#include <iostream>
#include <cstring>
#include <algorithm>
#include <limits.h>
#include <cmath>
#define REP(i,a,b) for(i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
/* ここからが本編 */
/*  */

int int_pow(int x,int n) {
   int res = 1;
   while(n > 0) {
      if(n&1) res = res * x;
      x = x * x;
      n >>= 1;
   }
   return res;
}
int main(void)
{
  int test = 1;
  int i,j,k,l;
  double xa,xb,ya,yb;
  cin >> xa >> ya >> xb >> yb;
  printf("%.9lf\n",ya+(yb-ya)*xa/(xa+xb));
  
  return 0;
}