#include <cstdio>

using namespace std;

int main()
{
    //freopen("file.in", "r", stdin);
    //freopen("file.out", "w", stdout);
    int t1,k1,t2,k2;
    scanf("%d%d%d%d",&t1,&k1,&t2,&k2);
    printf("%f",1.0*t1*t2/(t2*k1+t1*k2));
    return 0;
}