#include<cstdio>

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

    double t3 = double(t1*t2) / double( t2*k1 + t1*k2);

    printf("%lf\n", t3);

    return 0;
}