#include<bits/stdc++.h>
#define ll long long 
#define pb push_back
#define mp make_pair
#define mod 1000000007
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define F first
#define S second 
#define pii pair<int,int>
#define afor(i,a,b) for(int i=a;i<b;++i)
#define bfor(i,a,b) for(int i=a;i>=b;--i)
#define print(A,j,k) for(int ii=j;ii<k;ii++)cout<<A[ii]<<" ";cout<<"\n"
#define all(v) v.begin(),v.end()
#define ps cout<<"YES\n";
#define PI 3.14159265358979323846264338
#define CASES int t;cin>>t;while(t--)
//ans = a ? b : c; // to simplify: if (a) ans = b; else ans = c;
//if(x&1)-->odd else even.
//freopen("in.txt","r",stdin);
//freopen("out.txt","w",stdout);
using namespace std;
int main()
{
    long double t1,k1,t2,k2;
    cin>>t1>>k1>>t2>>k2;
    cout<<fixed<<setprecision(6)<<(t1*t2)/(k1*t2+k2*t1)<<"\n";
    return 0;
}