#include <bits/stdc++.h>
using namespace std;
struct nfsq {
    int a, b;
} q[200005];
bool cmp(nfsq x, nfsq y) { return x.b > y.b; }
int main() {
    int v, n;
    scanf("%d%d", &v, &n);
    for (int i = 1; i <= n; ++i) scanf("%d%d", &q[i].a, &q[i].b);
    sort(q + 1, q + 1 + n, cmp);
    long long tot = 0;
    for (int i = 1; i <= n; ++i) {
        if (v >= q[i].a) {
            v -= q[i].a;
            tot += q[i].a * q[i].b;
        } else {
            tot += v * q[i].b;
            break;
        }
    }
    printf("%lld", tot);
    return 0;
}