#include<algorithm>
#include<iostream>
using namespace std;
struct milk{
	int danjia,shuliang;
}a[10001];
int cmp(milk x,milk y){
	return x.shuliang <y.shuliang ;
}
int main(){
	int zongliang,renshu;
	cin>>zongliang>>renshu;
	for(int i=1;i<=renshu;i++){
		cin>>a[i].danjia >>a[i].shuliang ;
	}
	sort(a+1,a+renshu+1,cmp);
	int sum=0;
	for(int i=1;i<=renshu;i++){
		if(a[i].shuliang <zongliang){
			sum+=a[i].shuliang *a[i].danjia ;
			zongliang-=a[i].shuliang ;
		}
		else{
			sum+=a[i].danjia *zongliang;
			zongliang=0;
			break;
		}
	}
	cout<<sum;
	return 0;
}