#include <bits/stdc++.h>
using namespace std;
using ll=long long;
const ll MOD=(ll)1e9+7;
const ll MOD2=(ll)998244353;
const ll INF=(ll)1e18+10;
const double PI=acos(-1);

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
int dy[]={-1,1,0,0};
int dx[]={0,0,-1,1};

void solve(int cn) {
    int sx, sy, gx, gy;
    cin>>sx>>sy>>gx>>gy;
    double x;
    x=1.0*(gx-sx)*sy/(double)(sy+gy)+sx;
    printf("%.10f\n",x);
}


#define SINGLE
int main() {
#ifdef SINGLE
    solve(1);
#else
    int N;
    cin>>N;
    for(int i=0;i<N;++i){
        solve(i);
    }
#endif
    return 0;
}

#include<cstdio>
int a,b,c,d;
int main()
{
	scanf("%d%d%d%d",&a,&b,&c,&d);
	printf("%d",a*d-b*c);
	return 0;
}