#include<bits/stdc++.h>
using namespace std;
int n,m,money;
struct Node{
	int p,c;
}a[5005];
bool cmp(Node x,Node y){
	return x.p<y.p;
}
int main(){
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		cin>>a[i].p>>a[i].c;//cÊÇÊýÁ¿£¬pÊǼ۸ñ 
	}
	sort(a+1,a+m+1,cmp);
	for(int i=1;i<=m;i++){
		if(a[i].c>n){
			money+=a[i].p*n;
			break;
		}
		else {
			money+=a[i].c*a[i].p;
			n-=a[i].c;
		}
	}
	cout<<money;
	return 0;
}

/**************************************************************
	Problem: 1940
	User: chenpengxi
	Language: C++
	Result: Accepted
	Time:43 ms
	Memory:2116 kb
****************************************************************/