#include <bits/stdc++.h>
using namespace std;
struct w{
	int m;
	int k;
}q[26];
bool cmp(w w1,w w2){
return w1.k>w2.k;}
int a,b;
int main(){
	cin>>a>>b;
	for(int i=0;i<b;i++){
		cin>>q[i].m>>q[i].k;
	} 
	sort(q,q+b,cmp);
	int sum=0;
	int sun=0;
	for(int i=0;i<b;i++){
		if(q[i].m+sun>a)continue;
		sum+=q[i].m*q[i].k;
		sun+=q[i].m;
	}
	cout<<sum;
	return 0;
}