#include <cstdio>

long double t1, k1, t2, k2;

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

    long double ans = t1 / (k1 + (t1 / t2) * k2);
    printf("%.9Lf\n", ans);
    return 0;
}