#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;
        } else
            J = i;
    if (sum < V && sum + Water[J].a > V)
        ans += 1.0 * (V - sum) * Water[J].b;
    cout << ans;
    return 0;
}