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

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

	void add(unsigned int pos, unsigned int val){
		if(pos >= digit.size()) digit.insert(digit.end(), pos-digit.size()+1, 0);
		if(digit[pos] > (unsigned int)(0-val)) add(pos+1, 1);
		digit[pos] += val;
	}
	void sub(unsigned int pos, unsigned int val){
		if(digit[pos] < val) sub(pos+1, 1);
		digit[pos] -= val;
		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 -(){
		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++){
			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);
		}
	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){
	const char d[] = "0123456789abcdef";

	if(i.negative) out.put('-');
	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] );
		}
	}

	return out;
}

std::istream& operator >> (std::istream& in, integer& i){
	in >> std::ws;
	std::vector<char> vec;
	
	i.negative = 0;
	if(in.peek() == '-') i.negative = 1;
	if(in.peek() == '+' || in.peek() == '-') in.get();

	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++;	
	}

	return in;
}

int main(){
	integer i = 1;
	integer j;
	std::cout << "Enter a number.\nnote: use base 16. do not add 0x.\n";
	std::cin >> j;
	for(integer k = j; k > 0; k -= 1) i *= k;

	std::cout << j << "! = " << i;

	std::cin.ignore().get();
	return 0;
}