fractional_knapsack_greedy_algo.cpp

#include<bits/stdc++.h>
using namespace std;

bool compare(pair<int, int> p1, pair<int, int> p2) {
    double v1 = (double) p1.first / p1.second;
    double v2 = (double) p2.first / p2.second;

    return v1>v2;
}

int main() {

    int n;
    cin>>n;

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

    int w;
    cin>>w;
    
    sort(a.begin(), a.end(), compare);
    int ans = 0;

    for(int i=0; i<n; i++) {
        if(w > a[i].second) {
            ans += a[i].first;
            w -= a[i].second;
            continue;
        }

        double vperw = (double) a[i].first / a[i].second;
        ans += vperw*w;
        w = 0;
        break;
    }

    cout<<ans<<endl;
    return 0;
}