#include <iostream>

int main(){
    double x0, y0, x1, y1;
    scanf("%lf %lf %lf %lf", &x0, &y0, &x1, &y1);
    
    if(y0 == y1){
        printf("%.10f\n", y0);
        return 0;
    }

    printf("%.10f\n", x1 / (x0+x1) * (y0-y1) + y1);
}