#include "bits/stdc++.h"

using namespace std;

typedef long long ll;
typedef long double ld;

#define mp make_pair
#define pb push_back
#define FOR(i,a,x) for(int i=(a);i<(x);i++)
#define rep(i,n)	FOR(i,0,n)
#define xFOR(i,a,x) for(int i=(a);i<=(x);i++)
#define sor(v) sort(v.begin(),v.end())
#define rev(s) reverse(s.begin(),s.end())
#define lb(vec,a) lower_bound(vec.begin(),vec.end())
#define ub(vec,a) upper_bound(vec.begin(),vec.end())
#define uniq(vec) vec.erace(unique(vec.begin(),vec.end(),vec.end))
#define mp1(a,b,c) P1(a,P(b,c))
#define all(x) (x).begin(),(x).end()

#define debug(X) cout<<"["<<#X<<"]"<<X<<endl;



inline bool cheak(int x, int y, int xMax, int yMax) { return x >= 0 && y >= 0 && xMax > x && yMax > y; }
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<typename T> inline string toString(const T &a) { ostringstream oss; oss << a; return oss.str(); }
template<class T> inline T sqr(T x) { return x*x; }


const int dx[8] = { 1, 0, -1, 0, 1, 1, -1, -1 }, dy[8] = { 0, -1, 0, 1, -1, 1, -1, 1 };
//const int mod = 1000000007;
const int INF = 1e9;
const int NIL = -1;

#define MAX_N 10010
#define MAX_K 100
#define MAX_V 10000

double xa, ya;
double xb, yb;

int main() {
	cin >> xa >> ya;
	cin >> xb >> yb;

	//xb = -xb;

	cout << setprecision(10) << ya + (yb - ya)*xa / (xa + xb) << endl;
	
	return 0;
}