#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, x, y;
  cin >> n >> m;
  vector<pair<int, int>> v;
  for (int i = 0; i < m; i++) {
    cin >> x >> y;
    v.push_back({y, x});
  }
  sort(v.begin(), v.end());
  int ans = 0, counter = 0;
  for (int i = m - 1; i >= 0; i--) {
    if (counter + v[i].second <= n) {
      ans += v[i].first * v[i].second;
      counter += v[i].second;
    } else {
      ans += v[i].first * (n - counter);
      break;
    }
  }
  cout << ans << endl;
  return 0;
}
