muti map;

/*
                  ,     \    /      ,
                 / \    )\__/(     / \
                /   \  (_\  /_)   /   \
           ____/_____\__\@  @/___/_____\____
          |             |\../|              |
          |              \VV/               |
          |        ------___-------         |
          |__________Chuta Dragon___________|
           |    /\ /      \\       \ /\    |
           |  /   V        ))       V   \  |
           |/     `       //        '     \|
           `              V                '
        */
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define fastread() (ios_base:: sync_with_stdio(false),cin.tie(NULL))
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
void input(ll ara[],ll n)
{
    for(ll i=0; i<n; i++)cin>>ara[i];
}

#define eb emplace_back
int main() {


   multimap<ll,ll>m;

    ll n,m1,a,b;
   cin>>n>>m1;
     for(ll i=0; i<m1;i++)
     {
         cin>>b>>a;
         //m[a]=b;

         m.insert({a,b});

     }
  vector<pair<ll,ll>>v;
 // cout<<endl;
     for(auto x:m)
     {
         a=x.fi;
         b=x.se;
       //  cout<<a<<" "<<b<<endl;
          v.pb({a,b});

     }

 ll cnt=0,ans=0;
//  cout<<endl;
 for(ll i=m1-1; ;i--)
 { // cout<<v[i].fi<<" "<<v[i].se<<endl;
          if(v[i].se<n)
          {

                 ans=ans+(v[i].fi*v[i].se);
                 n=n-v[i].se;
                 if(i==0)break;
          }
          else
          {
              ans=ans+(n*v[i].fi);
              break ;
          }

 }

      cout<<ans<<endl;















}