#include <bits/stdc++.h> using namespace std; int main() { int x,a,y,b; float n; //为了输出保留小数点后两位，定义float n; cin >> x >> a >> y >> b; n = (b*y - a*x) / (b - a); printf("%.2lf",n); //cout << fixed << setprecision(2) << n; return 0; }