#include <bits/stdc++.h>
using namespace std;

const int N = 100010;

// 到第i个物品，总价值超过j的最大收益
int dp[30][N];

int main() { // 总钱数、个数
  int n, m;
  // 价格、重要度
  int v[30], w[30];
  cin >> n >> m;
  for (int i = 1; i <= m; i++)
    cin >> v[i] >> w[i];

  for (int i = 1; i <= m; i++) {
    for (int j = 0; j <= n; j++) {
      if (j < v[i]) {
        // 不够钱不买
        dp[i][j] = dp[i - 1][j];
      } else {
        dp[i][j] = max(dp[i - 1][j], dp[i - 1][j - v[i]] + v[i] * w[i]);
      }
    }
  }
  cout << dp[m][n] << endl;
  return 0;
}