#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) for(int i = (n) - 1; (i) >= 0; --i)
#define SZ(n) (int)(n).size()
#define ALL(n) (n).begin(), (n).end()
#define MOD LL(1e9 + 7)
#define INF 1000000
using namespace std;
typedef long long LL;
typedef vector<int> VI;
typedef pair<int, int> PI;

long double x[2];
long double y[2];

int main() {
	cin >> x[0] >> y[0];
	cin >> x[1] >> y[1];
	long double ans = (y[0] + (y[1] - y[0]) * x[0] / (x[1] + x[0]));
	cout << setprecision(12) << ans << endl;
	return 0;
}