268E - Playlist - CodeForces Solution

#include <iostream>
#include <vector>
#include <iomanip>
#include <algorithm>

using namespace std;


int main() {

    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int n; cin >> n;

    vector<pair<int, int>> data(n); // l, p
    for(int i = 0; i < n; i ++)
        cin >> data[i].first >> data[i].second;

    sort(data.begin(), data.end(),
         [](const pair<int, int>& data1, const pair<int, int>& data2){
        int l1 = data1.first, p1 = data1.second;
        int l2 = data2.first, p2 = data2.second;
        return p1 * l1 * (100 - p2) > p2 * l2 * (100 - p1);
    });

    long double liked_expected_len = 0.0, res = 0.0;
    for(int i = 0; i < n; i ++){
        long double l = data[i].first;
        long double p = (long double)data[i].second / 100.0;
        res += l;
        res += (1 - p) * liked_expected_len;
        liked_expected_len += p * l;
    }
    cout << fixed << setprecision(10) << res << '\n';


    return 0;
}