#include <cstdio>
#include <iostream>
#include <algorithm>

using namespace std;

const int maxn=5000+10;
struct node
{
	int w,v;
};
node a[maxn];
int n,m,ans;

bool comp(node i,node j);
int main()
{
	freopen("milk.in","r",stdin);
	freopen("milk.out","w",stdout);
	cin>>m>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>a[i].v>>a[i].w;
	}
	sort(a+1,a+1+n,comp);
	for(int i=1;i<=n;i++)
	{
		if(a[i].w<=m)
		{
			m-=a[i].w;
			ans+=a[i].v*a[i].w;
		}
		else
		{
			ans+=a[i].v*m;
			m=0;
			break;
		}
	}
	cout<<ans<<endl;
	return 0;
}
bool comp(node i,node j)
{
	if(i.v<=j.v) return true;
	else
	return false;
}