#include <stdio.h>

using namespace std;

int main()
{
    int t1,t2,k1,k2;
    scanf("%d %d %d %d",&t1,&k1,&t2,&k2);

    double sol = 1.0*t1*t2/(k1*t2+k2*t1);
    printf("%f",sol);
    return 0;
}