#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(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++) cin>>a[i].p>>a[i].c;
	sort(a+1,a+m+1,cmp);
	for(int i=1;i<=n;i++){
		if(a[i].c<n){
			money+=a[i].p*a[i].c;
            n=n-a[i].c;
		}
		else{
            money+=a[i].p*n;
            break;
        }
	}
	cout<<money;
	return 0;
}

/**************************************************************
	Problem: 1940
	User: liyunshuo
	Language: C++
	Result: Accepted
	Time:45 ms
	Memory:2116 kb
****************************************************************/