#include<bits/stdc++.h>
using namespace std;
struct hk{
	int dj;
	int sl;
}a[2000];
int m,n,s,t,c;
bool cmp(hk x,hk y){
	return x.dj<y.dj;
}
int main(){
	cin>>m>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i].dj>>a[i].sl;
	}
	sort(a+1,a+1+n,cmp);
	if(a[1].sl>=m) s=a[1].dj*m;
	else{
		s=s+a[1].dj*a[1].sl;
		c=c+a[1].sl;
		t=m-c;
		for(int i=2;i<=n;i++){
			if(a[i].sl>=t){
			s=s+a[i].dj*t;
			c=c+t;
			t=m-c;	
			}
			else{
				s=s+a[i].dj*a[i].sl;
				c=c+a[i].sl;
				t=m-c;
			}
			if(t==0) break;
		}
	}
	cout<<s;


	return 0;
}

/**************************************************************
	Problem: 1730
	User: zhuanghaoxiang
	Language: C++
	Result: Accepted
	Time:20 ms
	Memory:2092 kb
****************************************************************/