#include <iostream>
using namespace std;

int main()
{
    long long a, b, c, d;
    int r = 0, g;
    cin >> a >> b;
    cin >> c >> d;
    g = (a + b * (c - a + d) / (b + d)) - ((a + b * (c - a + d) / (b + d)) % b);
    cout << g << endl;
    return 0;
}

Edited by author 02.10.2020 16:52