#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
struct cow {
	long long p;
	long long num;
};
bool cmp(cow a,cow b) {
	return a.p<b.p;
}
int main() {
	long long n,m;
	cin>>n>>m;
	cow a[m+1];
	memset(a,0,sizeof(a));
	for(long long i=1; i<=m; i++) {
		cin>>a[i].p>>a[i].num;
	}
	sort(a+1,a+1+m,cmp);
	long long sum=0;
	for(long long i=1; i<=m; i++) {
		if(n>=a[i].num) {
			n-=a[i].num;
			sum+=a[i].num*a[i].p;
		}
		if(n<a[i].num) {
			sum+=n*a[i].p;
			break;
		}
//		cout<<n<<endl;
	}
	 cout<<sum;
	return 0;
}