#include <stdio.h>

int main() {
    int t1, t2, k1, k2;
    double result;
    
    scanf("%d%d%d%d", &t1, &k1, &t2, &k2);
    
    result = (double) (t1 *t2) / (t1 * k2 + t2 * k1);
    printf("%lf\n", result);
    
    return 0;
}