#include <bits/stdc++.h>
using namespace std;
int v, n, x, y, now;
unsigned long long int ans;
struct Tip {
    int vi, va;
} tip[200005];
bool cmp(Tip a, Tip b) { return a.va > b.va; }
int main() {
    scanf("%d%d", &v, &n);
    for (int i = 1; i <= n; i++) {
        scanf("%d%d", &x, &y);
        tip[i] = (Tip){ x, y };
    }
    sort(tip + 1, tip + 1 + n, cmp);
    for (int i = 1; i <= n; i++) {
        Tip ccc = tip[i];
        if (v - now < ccc.vi) {
            ans += ccc.va * (v - now);
            break;
        } else {
            ans += ccc.va * ccc.vi;
            now += ccc.vi;
        }
    }
    printf("%llu", ans);
}