#include<iostream>
#include<math.h>
#include<algorithm>
using namespace std;
typedef long long ll;
ll dp[26][30001];
ll v[26];
int w[26];
int total, m;
int main()
{

	cin >> total;
	cin >> m;
	for (int i = 1; i <= m; i++)
	{
		cin >> v[i];
		cin >> w[i];
	}
	for (int i = 1; i <= m; i++)
	{
		for (int j = 1; j <= total; j++)
		{
			if (j >=v[i])
			{
				dp[i][j] = max(dp[i - 1][j], dp[i - 1][j - v[i]] + v[i] * w[i]);
			}
		}
	}
	cout << dp[m][total];
	return 0;
}