#include<iostream>
#include<cmath>
#include<cstring>
#include<algorithm>
#include<ctime>
using namespace std;
struct milk
{
	long long price,wat;
}a[2000001];
bool cmp(milk a,milk b)
{
	return a.price<b.price;
}
int main()
{
	long long n,m,sum=0;
	cin>>n>>m;
	for(int i=1;i<=m;i++)
	{
		cin>>a[i].price>>a[i].wat;
	}
	sort(a+1,a+1+m,cmp);
	for(int i=1;i<=m;i++)
	{
		if(n<=0)    break;
		if(n>=a[i].wat)
		{
			n-=a[i].wat;
			sum+=(a[i].price*a[i].wat);
		}
		else
		{
			sum+=((n/a[i].wat)*a[i].price);
			n=0;
		}
		
	}
	cout<<sum;
	return 0;
}