#include<bits/stdc++.h>
#define int long long
#define QWQ cin.tie(0)->sync_with_stdio(false);
using namespace std;

struct Milk{
    int price, cnt;
}milk[123456789];

signed main(){
    QWQ
    int n, m;
    cin >> n >> m;
    for(int i = 1; i <= m; i++){
        cin >> milk[i].price >> milk[i].cnt;
    }
    sort(milk + 1, milk + m + 1, [](Milk a, Milk b)->bool{return a.price < b.price;});
    int resCnt = 0, resPrice = 0, k = 1;
    bool flag = true;
    while(flag){
        if(resCnt + milk[k].cnt <= n){
            resCnt += milk[k].cnt;
            resPrice += milk[k].price * milk[k].cnt;
        }
        else{
            resPrice += (n - resCnt) * milk[k].price;
            flag = false;
        }
        ++k;
    }
    cout << resPrice;

    return 0;
}