#include <bits/stdc++.h>   using namespace std;   int main() {     long long n, m;     cin >> n >> m;     vector<pair<long long, long long> > v;     for (int i = 0; i < m; i++) {       long long a, b;       cin >> b >> a;       v.push_back(make_pair(a, b));     }     sort(v.rbegin(), v.rend());     long long total = 0, i = 0, count = 0;     while (i < m && (count + v[i].second) <= n) {       total += v[i].first * v[i].second;       count += v[i].second;       i++;     }     if (i != m) total += v[i].first * (n - count);     cout << total << endl;   }   