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

int need,n;
long long moneysum;
struct farmer
{
	int money;
	int milk;
};
farmer a[5001];

bool cmp(farmer x,farmer y)
{
	return x.money<y.money;
}

int main()
{
	cin>>need>>n;
	
	for(int i=1;i<=n;i++)
	{
		cin>>a[i].money>>a[i].milk;
	}
	
	sort(a+1,a+n+1,cmp);
	
	int i=1;
	while(need)
	{
		if(a[i].milk>=0)
		{
			a[i].milk--;
			moneysum+=a[i].money;
			need--;
		}
		else i++;
	}
	
	cout<<moneysum;
	
	return 0;
}