#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int main(){
	freopen ("milk.in","r",stdin);
	freopen ("milk.out","w",stdout);
	int a,b,c,d[1009]={0},e[1009]={0},f,g,h,i,j,k[1009],l;
	cin>>a>>b;
	j=0;
	for (c=1;c<=b;c++)
	{
		cin>>d[c]>>e[c];
		k[c]=c;
	}
	for (f=1;f<=b;f++)
	{
		for (g=1;g<=b;g++)
		{
			if(d[f]<d[g])
			{
				h=d[f];
				d[f]=d[g];
				d[g]=h;
				l=k[f];
				k[f]=k[g];
				k[g]=l;
			}
		}
	}
	for (i=1;i<=a;i++)
	{
		if(a-e[k[i]]<0)
		{
			j=j+(a*d[i]);
			a=a-a;
		}
		else
		{
			a=a-e[k[i]];
			j=j+(d[i]*e[k[i]]);
		}
	}
	cout<<j;
	return 0;
}