#include <iostream>
#include <iomanip>
#include <vector>

#define DEC

struct integer{
private:
    bool negative;
    std::vector<char> digit;

    void add(unsigned int pos, unsigned int val){
        if(pos >= digit.size()) digit.insert(digit.end(), pos-digit.size()+1, 0);
#ifdef DEC
        if(digit[pos] + val >= 10){
            add(pos+1, 1);
            digit[pos] += val-10;
        }else digit[pos] += val;
#else
        if(digit[pos] > (unsigned int)(0-val)) add(pos+1, 1);
        digit[pos] += val;
#endif
    }
    void sub(unsigned int pos, unsigned int val){
#ifdef DEC
        if(digit[pos] < val){
            sub(pos+1, 1);
            digit[pos] -= val-10;
        }else digit[pos] -= val;
#else
        if(digit[pos] < val) sub(pos+1, 1);
        digit[pos] -= val;
#endif

        if(digit[pos] == 0 && pos+1 == digit.size()) digit.pop_back();
    }
    bool abs_greater(const integer& i) const{
        if(digit.size() != i.digit.size()) return digit.size() > i.digit.size();

        for(int k = digit.size()-1; k >= 0; k--)
            if(digit[k] != i.digit[k]) return digit[k] > i.digit[k];

        return 0;
    }

public:
    integer(int i){
        if(negative = i < 0) digit.push_back(-i);
        else digit.push_back(i);
    }
    integer(){}

    friend integer operator * (const integer&, const integer&);
    friend integer operator + (integer, integer);
    friend integer operator - (integer, integer);
    friend std::ostream& operator << (std::ostream&, const integer&);
    friend std::istream& operator >> (std::istream&, integer&);
    friend bool operator == (const integer&, const integer&);
    friend bool operator != (const integer&, const integer&);
    friend bool operator > (const integer&, const integer&);
    friend bool operator <= (const integer&, const integer&);
    friend bool operator < (const integer&, const integer&);
    friend bool operator >= (const integer&, const integer&);

    integer operator = (const integer& i){
        digit = i.digit;
        negative = i.negative;
        return i;
    }

    integer operator += (const integer& i){
        *this = *this + i;
        return *this;
    }
    integer operator -= (const integer& i){
        *this = *this - i;
        return *this;
    }

    integer operator *= (const integer& i){
        *this = *this * i;
        return *this;
    }

    integer operator /= (const integer& i){
        *this = *this / i;
        return *this;
    }

    integer operator -(){
        integer i = *this;
        i.negative = !i.negative;
        return i;
    }
};

integer operator + (integer a, integer b){
    if(a.negative == b.negative){
        for(unsigned int k = 0; k < b.digit.size(); k++)
            a.add(k, b.digit[k]);
    }else{
        if(b.abs_greater(a)) std::swap(a, b);
        for(unsigned int k = 0; k < b.digit.size(); k++)
            a.sub(k, b.digit[k]);
    }
    return a;
}
integer operator - (integer a, integer b){
    return a + (-b);
}

integer operator * (const integer& a, const integer& b){
    integer r;
    r.negative = a.negative ^ b.negative;

    for(unsigned int j = 0; j < a.digit.size(); j++)
        for(unsigned int k = 0; k < b.digit.size(); k++){
#ifdef DEC
            char val = a.digit[j]*b.digit[k];
            if(val >= 10) r.add(j+k+1, val/10);
            r.add(j+k, val%10);
#else
            unsigned long long int val = a.digit[j];
            val *= b.digit[k];

            if(val >> 32 > 0)
                r.add(j+k+1, val>>32);
            r.add(j+k, (unsigned int)val);
#endif

        }
    return r;
}

bool operator == (const integer& a, const integer& b){
    if(a.negative != b.negative || a.digit.size() != b.digit.size()) return false;

    for(int i = a.digit.size() -1; i >= 0; i--)
        if(a.digit[i] != b.digit[i]) return false;

    return true;
}
bool operator != (const integer& a, const integer& b){
    return !(a==b);
}
bool operator > (const integer& a, const integer& b){
    if(a.negative ^ b.negative) return !a.negative;
    return a.abs_greater(b) ^ a.negative;
}
bool operator < (const integer& a, const integer& b){
    return !(a>b || a==b);
}
bool operator >= (const integer& a, const integer& b){
    return a>b || a==b;
}
bool operator <= (const integer& a, const integer& b){
    return !(a>b);
}


std::ostream& operator << (std::ostream& out, const integer& i){
    if(i.negative) out.put('-');
#ifdef DEC
    for(int k = i.digit.size() -1; k >= 0; k--) out.put('0'+i.digit[k]);
    if(i.digit.size() == 0) out.put('0');
#else
    const char d[] = "0123456789abcdef";

    bool first = true;
    for(int k = i.digit.size() -1; k >= 0; k--){
        for(int j = 8-1; j >= 0; j--){
            char ch = (i.digit[k] >> j*4) & 0xf;
            if(ch != 0) first = false;
            if(!first || ch != 0) out.put( d[ch] );
        }
    }
#endif
    return out;
}

std::istream& operator >> (std::istream& in, integer& i){
    in >> std::ws;

    i.negative = 0;
    if(in.peek() == '-') i.negative = 1;
    if(in.peek() == '+' || in.peek() == '-') in.get();
#ifdef DEC
    while(1){
        char c = in.peek();
        if(c >= '0' && c <= '9') i.digit.push_back(in.get() - '0');
        else break;
    }
    for(int k = 0; k*2 < i.digit.size()-1; k++) std::swap(i.digit[k], i.digit[i.digit.size()-1-k]);
#else
    std::vector<char> vec;

    while(1){
        char c = in.peek(), i;
        if(c >= '0' && c <= '9') i = c - '0';
        else if(c >= 'a' && c <= 'f') i = c - 'a' + 10;
        else if(c >= 'A' && c <= 'F') i = c - 'A' + 10;
        else break;

        in.get();
        vec.push_back(i);
    }

    i.digit.clear();
    for(int k = vec.size() -1, m = 0, s = 0; k >= 0; k--){
        s += vec[k] << m*4;

        if(m == 7 || k == 0){
            i.digit.push_back(s);
            s = m = 0;
        }else m++;
    }
#endif
    return in;
}

int main(){
    integer i = 1;
    integer j;

    std::cout << "Enter a decimal number.\n";
    std::cin >> j;

    for(integer k = j; k > 0; k -= 1) i *= k;

    std::cout << j << "! = " << i;
    std::cin.ignore().get();
    return 0;
}