#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("milk.in");
ofstream fout("milk.out");
int n,m,money=0;
class farmer
{
public:
	int price,milk;
}num[5010];
int cmp(farmer x,farmer y)
{
	if(x.price>y.price)
		return 0;
	else
		return 1;
}
int main()
{
	fin>>n>>m;
	for(int i=1;i<=m;i++)
	{
		fin>>num[i].price>>num[i].milk;
	}
	sort(num+1,num+1+m,cmp);
	for(int i=1;i<=m;i++)
	{
		if(num[i].milk>=n)
		{
			money+=num[i].price*n;
			break;
		}
		else
		{
			n-=num[i].milk;
			money+=num[i].price*num[i].milk;
		}
	}
	fout<<money<<endl;
	
	fin.close();
	fout.close();
	return 0;
}