#include<bits/stdc++.h>
using namespace std;
struct jc
{
	int dian;
	int liang; 
}geshu[2000010];
bool pan(jc a, jc b)
{
	if(a.dian<b.dian)
	{
		return a.dian < b.dian;
	}
	return a.dian > b.dian;
}
int main(void)
{
	int n, m;
	scanf("%d%d", &n, &m);
	for(int i = 1;i <= m;i++)
	{
		scanf("%d%d", &geshu[i].dian, &geshu[i].liang);
	}
	long long ans = 0, point = 0;
	sort(geshu, geshu+m, pan);
	for(int i = 1;i <= m;i++)
	{
		if(point >= n)
		{
			break;
		}
		if(geshu[i].liang+point>n)
		{
			ans+=(n-point)*geshu[i].dian;
			break;
		}
		else
		{
			ans+=geshu[i].dian*geshu[i].liang;
			point += geshu[i].liang;	
		}
	}
	printf("%lld", ans);
	return 0;
}