#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MAXN = 2e5 + 7;
struct water {
    int a, b;
} Water[MAXN];
bool compare(water x, water y) { return x.b > y.b; }
ll minx(ll x, ll y) { return x > y ? y : x; }
ll V, n, ans = 0;
int main() {
    cin >> V >> n;
    for (int i = 0; i < n; i++) cin >> Water[i].a >> Water[i].b;
    sort(Water, Water + n, compare);
    int cnt = 0;
    for (int i = 0; i < n; i++) {
        if (V == 0)
            break;
        if (V - Water[i].a > 0) {
            ans += (minx(Water[i].a, V)) * Water[i].b;
            V -= Water[i].b;
        }
        if (i == n - 1)
            break;
    }
    cout << ans;
    return 0;
}