#include<bits/stdc++.h>
using namespace std;
int n,m,money;
struct Node{
	int p,c;
}a[5001];
bool cmp(Node x,Node y){
	return x.p<y.p;
}
int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++)
		scanf("%d%d",&a[i].p,&a[i].c);
	sort(a+1,a+m+1,cmp);
	for(int i=1;i<=m;i++){
		if(n<a[i].c){
			money+=a[i].p*n;
			break;
		}
		money+=a[i].p*a[i].c;
		n-=a[i].c;
	}
	printf("%d",money);
}
/**************************************************************
	Problem: 1940
	User: chenshuo
	Language: C++
	Result: Accepted
	Time:42 ms
	Memory:2116 kb
****************************************************************/