#include<iostream>
#include<algorithm>
using namespace std;
struct milk{
	int many;
	int much;
}; 
bool cmp(milk x,milk y){
	if(x.much!=y.much)
	return x.much<y.much;
	else
	return x.many>y.many;
}
int n,m,ans,i;
int main(){
    cin>>n>>m;
    milk a[m+10];
    for(i=0;i<m;i++) cin>>a[i].much>>a[i].many ;
    sort(a,a+m,cmp);
    i=0;
    while(n>=a[i].many){
    	ans+=a[i].many *a[i].much ;
    	n-=a[i].many;
    	i++;
	}
    ans=ans+n*a[i].much ;
    cout<<ans;
}