using namespace std;
signed main(){
	double x,a,y,b;
	cin>>x>>a>>y>>b;
	double ans = ( a*x - b*y )/( a - b );
	printf( "%.2f",ans);
	return 0;
}

#include<bits/stdc++.h> using namespace std; signed main(){ double x,a,y,b; cin>>x>>a>>y>>b; double ans = ( ax - by )/( a - b ); printf( "%.2f",ans); return 0; }