一本通1005

//qf
#include <bits/stdc++.h>
#define f(i,j,n) for(register int i=j;i<=n;i++)
using namespace std;
typedef long long ll;
inline ll read() {
	ll x=0;
	int f=1;
	char ch=getchar();
	while(!isdigit(ch)) {
		if(ch=='-') f=-1;
		ch=getchar();
	}
	while(isdigit(ch)) x=(x<<1)+(x<<3)+(ch-48),ch=getchar();
	return x*f;
}
signed main() {
	int x=read(),a=read(),y=read(),b=read();
    cout<<fixed<<setprecision(2)<<(double)(b*y-a*x)/(b-a)<<endl;
    return 0;
}