//費用最大為:2e9; 貪心
#include <iostream>
#include <algorithm>
using namespace std;
const int PRODUCER_MAX = 5005;

struct part {
    int price, tot;
};
int part_tot, producer_tot;
part p[PRODUCER_MAX];

void read() {
    cin >> part_tot >> producer_tot;
    for (int i = 1; i <= producer_tot; i++) {
        cin
 
 >> p[i].price >> p[i].tot;
    }
}

bool cmp(const part& p1, const part& p2) {
    if (p1.price < p2.price) return true;
    else return false;
}

void solve() {
    sort(p + 1, p + producer_tot + 1, cmp);
    int tmp = part_tot;
    int cost = 0;
    for (int i = 1; i <= producer_tot; i++) {
        if
 
 (tmp <= p[i].tot) {
            cost += tmp*p[i].price;
            break;
        }
        else {
            cost += p[i].tot * p[i].price;
            tmp -= p[i].tot;
        }
    }
    cout << cost << '\n';
}

int main() {
    read();
    solve();
    return 0;
}