#include <iostream>
#include <algorithm>
using namespace std;

struct node {
    long long w, v;
} a[200005];

int cmp(node x, node y) { return x.v > x.v; }

unsigned long long ans;

int main() {
    int l, n;
    cin >> l >> n;
    for (int i = 1; i <= n; i++) {
        cin >> a[i].w >> a[i].v;
    }
    sort(a + 1, a + 1 + n, cmp);
    for (int i = 1; i <= n; i++) {
        if (l >= a[i].w) {
            ans += a[i].w * a[i].v;
            l -= a[i].w;
        } else {
            cout << ans + a[i].v * l;
            return 0;
        }
    }
    cout << ans;
    return 0;
}