#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn = 2e5 + 10;
typedef struct {
    int a;
    int b;
} goods;
goods arr[maxn];
int n, V;
bool cmp(goods x, goods y) { return x.b > y.b; }
int main() {
    cin >> V >> n;
    for (int i = 0; i < n; i++) cin >> arr[i].a >> arr[i].b;
    sort(arr, arr + n, cmp);
    ll re = 0;
    for (int i = 0; i < n; i++) {
        if (V > arr[i].a) {
            V -= arr[i].a;
            re += (arr[i].a * arr[i].b);
        } else if (V <= arr[i].a) {
            re += (V * arr[i].b);
            cout << re;
            return 0;
        }
    }
    cout << re;
    return 0;
}