#include <iostream>
#include <algorithm>
using namespace std;
const int MAX = 2e5 + 10;
struct water {
    int a;
    int b;
    int v;
    bool operator<(const struct water &p) { return b > p.b; }
} t[MAX];
int main() {
    int V, n;
    cin >> V >> n;
    for (int i = 1; i <= n; i++) {
        cin >> t[i].a >> t[i].b;
        t[i].v = t[i].a * t[i].b;
    }
    sort(t + 1, t + n + 1);
    int sum = 0, value = 0;
    for (int i = 1; i <= n; i++) {
        if (sum + t[i].a <= V) {
            sum += t[i].a;
            value += t[i].v;
        } else {
            value += (V - sum) * t[i].b;
            break;
        }
    }
    cout << value;
    return 0;
}