fracks.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);
    double ans = 0;
    for(int i=0;i<n;i++){
        if(w>=a[i].second){
            ans+=a[i].first;
            w-=a[i].second;
            continue;
        }
        double vw = (double) a[i].first/a[i].second;
        ans += vw * w;
        w=0;
        break;
    }
    cout << ans << endl;
}