#include <iostream>
#include <cstdio>

using namespace std;

int vl, vr, d, w;

int main() {
    // freopen("run.in", "r", stdin);
    // freopen("run.out", "w", stdout);

    scanf("%d%d%d%d", &vl, &vr, &d, &w);

    printf("%.15lf\n", 1.0 * d / (vl + vr) * w);

    return 0;
}