#include<iostream>
#include<algorithm>
using namespace std;
int n,m,ans;
struct node{
	int x,y;
}a[2000010];
bool cmp(node x,node y){
	return x.x<y.x;
} 
int main(){
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		cin>>a[i].x>>a[i].y;
	}
	sort(a+1,a+1+n,cmp);
	for(int i=1;i<=n;i++){
		if(n>=a[i].y){
			n-=a[i].y;
			ans+=a[i].x*a[i].y;
		}else{
			ans+=a[i].x*n;
			n=0;
		}
		if(n<=0){
			break;
		}
	}
	cout<<ans;
	return 0;
}