#include<bits/stdc++.h>
using namespace std;
int n,m,money[100001],milk[100001],ans,amount;

int main()
{
	cin>>m>>n;
	for(int i=1;i<=n;i++)
		cin>>money[i]>>milk[i];
	
	for(int i=1;i<n;i++)
		if(money[i]>money[i+1])
		{
			swap(money[i],money[i+1]);
			swap(milk[i],milk[i+1]);
		}
	
	for(int i=1;i<n;i++)
		for(int j=1;j<=milk[i];j++)
		{
			amount+=j;
			ans+=money[i];
			if(amount==m)break;
		}
	cout<<ans;
	return 0;
}