68B - Energy exchange - CodeForces Solution

#include <bits/stdc++.h>

using namespace std;
#define ll  long long
int n;
double k;
double a[10005];
bool check (double mid)
{
    double need=0;
    double have=0;
    for(int i=0;i<n;i++)
    {
        if (a[i]>=mid)
            have+= a[i] - mid;
        if (a[i]<mid)
            need += mid - a[i];
    }
    have = have - (have*k)/100.0;
    return have>=need;
}
void solve() {
    cin>>n;
    cin>>k;
    for(int i=0;i<n;i++)
        cin>>a[i];

    double maxi=0;
   double l =0;
   double r= 1e9;
    for(int i = 0; i < 400; i++){
       double mid = (l+r)/2.0;
       if (check(mid))
       {
           maxi= max(maxi,mid);
           l = mid;
       }
       else
           r = mid;


   }
    cout << fixed << setprecision(9)<<maxi;
}

int main() {
    solve();
    return 0;
}