
#include<bits/stdc++.h>

using namespace std;


#define ff              first
#define ss              second
#define int             long long
#define pb              push_back
#define mp              make_pair
#define pii             pair<int,int>
#define vi              vector<int>
#define mii             map<int,int>
#define pqb             priority_queue<int>
#define pqs             priority_queue<int,vi,greater<int> >
#define setbits(x)      __builtin_popcountll(x)
#define zrobits(x)      __builtin_ctzll(x)
#define all(a)      (a).begin(),(a).end()

#define ps(x,y)         fixed<<setprecision(y)<<x
#define mk(arr,n,type)  type *arr=new type[n];
#define w(x)            int x; cin>>x; while(x--)
#define REP(i,n) for(i=0;i<n;i++)
#define FIO             ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
mt19937                 rng(chrono::steady_clock::now().time_since_epoch().count());
const long long   M = 1e9 + 7;

void c_p_c()
{
#ifndef  ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif
}

int mod(int x) {
	return (x % M + M) % M;
}
int  add(int a, int b)
{
	return mod(mod(a) + mod(b));
}
void solve()
{
	double x, y, a, b;
	cin >> x >> y >> a >> b;
	cout << fixed << setprecision(11);

	cout << x + (a * y - x * y) / (b + y);
}

int32_t main()
{

	c_p_c();

	FIO;

	int tt = 1;
	solve();



}






#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
long double pi = 2 * acos(0.0);
const ll mod = 998244353;

void solve() {
    ll x1, y1, x2, y2;
    cin >> x1 >> y1 >> x2 >> y2;
    ld x = (x2 - x1) * y1 * 1.0;
    x = (ld)x / (y2 + y1);
    cout << fixed << setprecision(10) << x1 + x << "\n";
}    

int main() {
    ios_base :: sync_with_stdio(false); 
    cin.tie(nullptr);   
    ll t = 1;
    //cin >> t;
    for(ll i = 1; i <= t; i++) {
        solve();
    }
    return 0;
}