#include<iostream>
#include<algorithm>
using namespace std;
pair<int, int> far[2000007];
int main() {
	long long want,n,money=0;
	scanf("%lld %lld",&want,&n); 
	for(int i=0;i<n;++i) {
		scanf("%d %d",&far[i].first,&far[i].second);
	}
	sort(far,far+n);
	for(int i=0;;++i) {
		if(want>=far[i].second) {
			want-=far[i].second,money+=far[i].first*far[i].second;
		}
		else {
			money+=want*far[i].first;
			break;
		}
	}
	printf("%lld\n",money);
	return 0;
}