#include <iostream>
#include <algorithm>
using namespace std;
const int N = 2e5 + 5;
long long int V, n;
struct nfsq {
    long long int a;
    long long int b;
} a[N];
bool cmp(nfsq a, nfsq b) { return a.b > b.b; }
int main() {
    cin >> V >> n;
    for (int i = 1; i <= n; i++) cin >> a[i].a >> a[i].b;
    sort(a + 1, a + n + 1, cmp);
    int ans = 0;
    for (int i = 1; i <= n; i++) {
        if (V > a[i].a) {
            V -= a[i].a;
            ans += a[i].a * a[i].b;
        } else {
            ans += V * a[i].b;
            break;
        }
    }
    cout << ans << endl;
    return 0;
}