C++ :

#include<cstdio>
#include<iostream>
#include<cstdlib>
#include<algorithm>
using namespace std;
struct pos
{
	int cost;
	int num;
}f[10001];
int cmp(pos i,pos j)
{
	if(i.cost==j.cost)
	{
		return i.num>j.num;
	}
	else
	{
		return i.cost<j.cost;
	}
}
int main()
{
	int m,n;
	cin>>m>>n;
	for(int i=1;i<=n;++i)
	{
		cin>>f[i].cost>>f[i].num;
	}
	sort(f+1,f+n+1,cmp);
	
	long long ans=0;
	for(int i=1;i<=n;++i)
	{
		while(m!=0&&f[i].num!=0)
		{
			--m;
			ans=ans+f[i].cost;
			--f[i].num;
		}
		if(m==0)
		{
			break;
		}
	}
	
	cout<<ans;
	return 0;
	
}