длинная арифметика -

#include <bits/stdc++.h>

using namespace std;

typedef vector<int> num;
int const base = 1000 * 1000 * 1000;



class Num
{

public:
    vector<int>m;
    read()
    {
        num a;
        string s;
        cin >> s;
        int size = s.size();
        for (int i=(int)s.length(); i>0; i-=9)
            if (i < 9)
                this->m.push_back (atoi (s.substr (0, i).c_str()));
            else
                this->m.push_back (atoi (s.substr (i-9, 9).c_str()));
        //this->show();

    }
    show()
    {
        cout << (this->m.empty() ? 0 : this->m.back());
        int size = this->m.size();
        for (int i = size - 2; i > -1; --i)
        {
            cout << this->m[i];
        }

    }
    vector<int> substract(vector<int> a, vector<int> b)
    {
        // a>=b
        int carry = 0;
        for (int i = 0; i < b.size() || carry; ++i)
        {
            a[i] -= carry + (i < b.size() ? b[i] : 0);
            carry = a[i] < 0;
            if (carry){
                    a[i] += base;

                    }
        }
        while (a.size() > 1 && a.back() == 0)a.pop_back();
        return a;
    }

    Num  operator-(const Num& b)
    {
        Num s;
        if (this->m.size() > b.m.size())
        {

            s.m = substract(this->m, b.m);
            return s;
        }
        if (this->m.size() == b.m.size())
        {
            bool f = 0;
            for (int i = this->m.size() - 1; i > -1; --i)
            {
                if (this->m[i] < b.m[i])
                {
                    f = 1;
                    break;
                }
            }
            if (!f)
            {
                s.m = substract(this->m, b.m);
                return s;

            }

            s.m =  substract(b.m, this->m);
            s.m[s.m.size() - 1] *= -1;
            return s;
        }
        if (this->m.size() < b.m.size())
        {

            s.m =  substract(b.m, this->m);
            s.m[s.m.size() - 1] *= -1;
            return s;
        }

    }

};







int main()
{
    Num a;
    a.read();
    Num b;
    b.read();
    Num c = a - b;
     c.show();

    return 0;
}