#include <stdio.h>
#include <algorithm>
const int N = 2 * 1e5 + 10;
int V, n;
struct arry {
    int a;
    int b;
} m[N];
bool comp(arry a, arry b) { return a.b > b.b; }
int main() {
    int ans = 0, t = 0;
    scanf("%d %d", &V, &n);
    for (int i = 0; i < n; i++) scanf("%d %d", &(m + i)->a, &(m + i)->b);
    std::sort(m, m + n, comp);
    for (int i = 0; i < n; i++) {
        if (((m + i)->a + t) <= V) {
            ans += (m + i)->a * (m + i)->b;
            t += (m + i)->a;
        } else {
            ans = ans + (V - t) * (m + i)->b;
            break;
        }
    }
    printf("%d", ans);
    return 0;
}