#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
int main()
{
    LL vl, vr;
    LL d, vbee;
    scanf("%lld%lld%lld%lld", &vl, &vr, &d, &vbee);
    long double ans = 1.0 * d / (vl + vr) * vbee;
    cout << fixed << setprecision(8) << ans << endl;
    return 0;
}