#include<stdio.h>

void run(void){
  int l,r;
  scanf("%d%d",&l,&r);
  int d;
  scanf("%d",&d);
  int w;
  scanf("%d",&w);
  printf("%.7lf\n",(double)d/(l+r)*w);
  return;
}

int main(void){
  run();
  return 0;
}