简单易懂

#include<bits/stdc++.h>
#include <iomanip>
using namespace std;
int x,y,n,p;
double a,b;

int main(){
    cin>>x>>y>>n>>p;
    if(p>=x){a=p-y;}
    else{a=p;}
    b=p*0.1*n;
    if(a>b){std::cout << std::fixed << std::setprecision(2) << b << std::endl;;}//不用管这一大串是什么
    if(a==b){ std::cout << std::fixed << std::setprecision(2) << a << std::endl;;}
    if(a<b){std::cout << std::fixed << std::setprecision(2) << a << std::endl;;}
    return 0;
}