#include <iostream>
#include<iomanip>
using namespace std;

int main() {
   int n,x,a,b;
   cin >> n >> x >> a >> b;
   double expectation =n*a*x/100.0+n*b*(100-x)/100.0;  
   cout << fixed<< setprecision(2) << expectation << endl;
}

期望的算法

每种可能的收益乘概率相加