#include<bits/stdc++.h>
using namespace std;
struct farmer{
	int p,a;
}a[1001];
bool cmp(farmer a,farmer b){
	if(a.p != b.p)
		return a.p < b.p;
	else return a.a > b.a;
}
int main(){
	int n,m,money = 0;
	cin>>n>>m;
	for(int i = 1;i <= m;i++){
		cin>>a[i].p>>a[i].a;
	}
	sort(a+1,a+n+1,cmp);
	int i = 1;
	while(n >= 0){
		money += a[i].a*a[i].p;
		n -= a[i].a;
		i++;
	}
	cout<<money+n*a[i-1].p;
	return 0;
}