#include<bits/stdc++.h>
using namespace std;
struct node{
	int c,b;
}a[50115];
bool cmp(node p,node q){
	if(p.c>q.c){
		return false;
	}
	return true;
}
int main(){
	long long n,m,sum =0;
	cin >> n  >> m;
	for(int i = 1;i <= m;i ++){
		cin >> a[i].c >> a[i].b;
	}
	sort(a + 1, a + m + 1, cmp);
	for(int i = 1;i <= m;i ++){
		if(n >= a[i].b) {
			n -= a[i].b;
			sum += a[i].b * a[i].c;
		}else{
			sum += a[i].c * n;
			break;
		}
		
	}
	cout << sum;
	return 0;
}

RE了第6个点。