#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e5 + 7;
struct water {
    int a, b;
} Water[MAXN];
bool compare(water x, water y) { return x.b > y.b; }
int V, n, sum = 0, ans = 0, J;
int main() {
    cin >> V >> n;
    for (int i = 0; i < n; i++) cin >> Water[i].a >> Water[i].b;
    sort(Water, Water + n, compare);
    for (int i = 0; i < n; i++) {
        if (sum + Water[i].a < V) {
            sum += Water[i].a;
            ans += Water[i].a * Water[i].b;
        }
        if (sum == V || i + 1 == n) {
            cout << ans;
            return 0;
        } else if (sum + Water[i + 1].a > V) {
            ans += 1.0 * (V - sum) * Water[i + 1].b;
            cout << ans;
            return 0;
        }
    }
}