#include<iostream>
#include<algorithm>
#define N 10000

struct Node{
	int k, n;
	bool operator < (Node s){
		return k<=s.k;
	}
}a[N];

int  n,m,ans=0,k=1;
int main() {
	std::cin >> n >> m;
	for (int i = 1; i <=m; i++) std::cin >> a[i].k >> a[i].n;
	std::sort(a+1, a +1+ m);
	while (n > 0) {
		if (a[k].n <= n) {
			n -= a[k].n;
			ans +=( a[k].n*a[k].k);
		}
		else {
			ans += (n*a[k].k);
			break;
		}
		k++;
	}
	std::cout << ans<<std::endl;
	return 0;
}