#include<iostream>
#include<algorithm>
using namespace std;
const int N = 100010;
int m, n, niunai;
long long sum=0;
int a[N], b[N];
void f()
{
	if (m == 0 && n == 0)
	{
		sum = 0;
		return;
	}
	sort(a + 1, a + m + 1);
	for (int i = 1; i <= m; i++)
	{
		for (int j = 1; j <= b[a[i]]; j++)
		{
			sum += a[i];
			niunai++;
			if (niunai == n)return;
		}
	}
}
int main()
{
	cin >> n >> m;
	for (int i = 1; i <= m; i++)
	{
		int t;
		cin >> a[i];
		cin >> t;
		b[a[i]] += t;
	}
	f();
	cout << sum;
}