#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <functional>
#include <stack>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
const int maxn = 2e5 + 5;
struct water {
    ll volume;
    ll value;
};
water w[maxn];
bool cmp(water A, water B) {
    return A.value > B.value;  //价值降序
}

int main() {
    ll V, n;
    cin >> V >> n;
    ll ans = 0;
    for (int i = 1; i <= n; i++) {
        cin >> w[i].volume >> w[i].value;
    }
    sort(w + 1, w + n + 1, cmp);
    while (V) {
        for (int i = 1; i <= n; i++) {
            if (V > w[i].volume) {
                V -= w[i].volume;
                ans += w[i].volume * w[i].value;
            } else {
                ans += V * w[i].value;
                V = 0;
            }
        }
    }
    cout << ans << endl;
    system("pause");
    return 0;
}