#include<bits/stdc++.h>
using namespace std;
struct cow{
	unsigned long long d,l;
	
}a[60000];
bool cmp(cow x,cow y){
	return x.d<y.d;
	return x.l>y.l;
}
unsigned long long n,m;
int main(){
	cin>>n>>m;
	for(int i=0;i<m;i++){
		cin>>a[i].d>>a[i].l;
	}
	unsigned long long ans=0;
	sort(a,a+m,cmp);
	for(int i=0;i<m;i++){
		if(n>a[i].l){
			ans+=a[i].d*a[i].l;
			n-=a[i].l;
		}
		if(n<=a[i].l){
			ans+=n*a[i].d;
			n=0;
		}
	}
	cout<<ans;
	return 0;
}