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