#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <map>
#include <set>
#include <cstdio>
#include <cmath>
#define rep(i,l,n) for(int i=l;i<n;i++)
#define rer(i,l,n) for(int i=l;i<=n;i++)
#define all(a) a.begin(),a.end()
#define o(a) cout<<a<<endl
#define pb(a) push_back(a)
#define mk(a,b) make_pair(a,b)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pii;

int main(){
	double xa,xb,ya,yb,yp;
	cin>>xa>>ya>>xb>>yb;
	yp=ya+(yb-ya)*xa/(xa+xb);
	printf("%.8lf",yp);
}