#include<stdio.h>
main(){
    int A,B,X,Y;
    scanf("%d%d%d%d",&A,&B,&X,&Y);
    double ratio = (double)B/A;//1:B/A
    double ans,y2 = Y/ratio;
    if(X > y2){
        ans = Y+y2;
    }else{
        ans = X+X*ratio;
    }
    printf("%lf\n",ans);
}