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



/**************************************************************
	Problem: 1730
	User: hulaoshi
	Language: C++
	Result: Accepted
	Time:18 ms
	Memory:2092 kb
****************************************************************/