#include<bits/stdc++.h>
using namespace std;

long long X1, Y1, X2, Y2;

int main(){
    scanf("%lld%lld%lld%lld", &X1, &Y1, &X2, &Y2);
    printf("%.12Lf", (static_cast<long double>(X1) * Y2 + X2 * Y1) / (X1 + X2));
    return 0;
}