#include <bits/stdc++.h>
const int inf = 0x3f3f3f3f;
const long long int INF = 0x3f3f3f3f3f3f3f3f;
using namespace std;
inline void INP() {}
long long int n, m;
vector<pair<long long int, long long int> > v;
void solve() {
  cin >> n >> m;
  long long int mn = INF;
  long long int mx = 0;
  for (long long int i = 0; i < m; i++) {
    long long int x, y;
    cin >> x >> y;
    v.push_back({-1 * y, x});
    mn = min(mn, y);
    mx = max(mx, y);
  }
  sort(begin(v), end(v));
  long long int ans = 0;
  for (long long int i = 0; i < m; i++) {
    if (v[i].second <= n) {
      ans += (-1 * v[i].first) * v[i].second;
      n -= v[i].second;
    } else {
      ans += (-1 * v[i].first) * n;
      n = 0;
    }
    if (n == 0) break;
  }
  cout << ans << '\n';
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
