#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<vector>
#include<set>
#include<map>

using namespace std;

#define x first
#define y second

double t1,t2,k1,k2;

int main (){
    
    /*dist / v1 = t1
    dist / v2 = t2
    
    v1  = v2 * t2 / t1
    
    dist / (v2 * t2 / t1 * k1 + v2 * k2)
    
    v2 * (t2 * k1 / t1 + k2)*/
    
    scanf("%lf%lf%lf%lf", &t1,&k1,&t2,&k2);
    printf("%.4lf\n", t2 / (t2 * k1 / t1 + k2));
    
    return 0;
}