#include<bits/stdc++.h> 
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace std;
using namespace __gnu_pbds;  
#define ordered_set tree<ll, null_type , less<ll> , rb_tree_tag , tree_order_statistics_Node_update> 
#define ll long long
#define ull unsigned long long
#define pb push_back
#define inf 3e18
#define mk make_pair
#define ld long double
#define mod 1000000007
#define fi first
#define se second
#define fastIO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define test ll t; cin>>t; while(t--)
#define setbits __builtin_popcount
#define endl '\n'
#define LOCAL
#define sim template < class c
#define ris return * this
#define dor > debug & operator <<
#define eni(x) sim > typename \
enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; };
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
~debug() { cerr << endl; }
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i));}
sim, class b dor(pair < b, c > d) {
    ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
    *this << "[";
    for (auto it = d.b; it != d.e; ++it)
        *this << ", " + 2 * (it == d.b) << *it;
    ris << "]";
}
#else
sim dor(const c&) { ris; }
#endif
};
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
int main()
{
	ld x,y,z,w;
	cin>>x>>y>>z>>w;
	ld ans,val,val1;
	val=x*w+y*z;
	val1=w+y;
	ans=val/val1;
	cout<<fixed<<setprecision(9)<<ans<<endl;
}
#include<bits/stdc++.h>
using namespace std;
int main(){
double res;
double x1,y1,x2,y2;
cin>>x1>>y1>>x2>>y2;
 printf("%.10f",(y1*x2+x1*y2)/(y1+y2));
return 0;}
