ПЛОЩАДЬ

#include <bits/stdc++.h>

using namespace std;

int main(){
    double a, b, m, k;
    cin >> a >> b >> m >> k;

    if (a * m + b * k <= a * b) {
        cout << m * k << endl;
        return 0;
    }

    double x = b;
    double y = a;

    double S = x * y / 2;

    if (x > m) {
        double my = (a * b - a * m) / b;
        double s1 = (x - m) * my / 2;
        S -= s1;
    }

    if (y > k) {
        double xk = (a * b - b * k) / a;
        double s2 = (y - k) * xk / 2;
        S -= s2;
    }

    cout << ceil(S) << endl;
}