#include <stdio.h>
int main(){
    int weight, speed;
    float balance, tilt;
    float staiityIndex;
    
    scanf("%d", &weight) != 1 ||
    scanf("%d", &speed) != 1 ||
    scanf("%f", &balance) != 1 ||
    scanf("%f", &tilt) != 1 || {
    printf("Invalid input");
    return 0;
    }
    
    if (weight <= 0 || speed<= 0 || balance <= 0 || tilt <= 0){
    printf("Invalid input");
    return 0;
    }
    staiityIndex = (weight / (speed + tilt)) * (balance / 2);
    printf("%.2f", staiityIndex);
    return 0;
}