src2020/time/timecin.cpp

#include <iostream>
using namespace std;
int main()
{
    long long anss, ansf, chus = 0, chuf = 0;
    int s, f;
    cin >> anss >> ansf;
    while (cin >> s >> f) {
        chuf += ansf - f;
        if (chuf < 0) {
            chus--;
            chuf += 60;
        }
        if (chuf > 59) {
            chus++;
            chuf -= 60;
        }
        chus += anss - s;
    }
    cout << chus << ":";
    if (chuf < 10)
        cout << "0";
    cout << chuf;
}