USACO-T010: Mixing Milk

/*
ID: mickyta1
TASK: milk
LANG: C++
*/

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

typedef pair<int, int> pii;

const int N = 5000;

pii seller[N + 1];

int main(){

    freopen("milk.in", "r", stdin);
    freopen("milk.out", "w", stdout);

    int trMilk, nSell;
    scanf("%d%d", &trMilk, &nSell);
    for(int i = 1; i <= nSell; ++i){
        scanf("%d%d", &seller[i].first, &seller[i].second);
    }
    sort(seller + 1, seller + nSell + 1);
    int sum = 0;
    for(int i = 1; i <= nSell && trMilk > 0; ++i){
        int price = seller[i].first;
        int volume = seller[i].second;
        if(volume > trMilk){
            sum += price * trMilk;
            trMilk = 0;
        } else {
            sum += price * volume;
            trMilk -= volume;
        }
    }
    cout << sum << '\n';

    fclose(stdin);
    fclose(stdout);

    return 0;
}