vector + pair+ reverse sort

/*
                  ,     \    /      ,
                 / \    )\__/(     / \
                /   \  (_\  /_)   /   \
           ____/_____\__\@  @/___/_____\____
          |             |\../|              |
          |              \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];
   //  if(ara[i]>mx)mx=ara[i];
   //  if(ara[i]<mn)mn=ara[i];
   // m[ara[i]]++;

    }
}

#define eb emplace_back
int main()
{
    ll n,m,a,b,i;
    cin>>n>>m;
    vector<pair<ll,ll>>v;

    while(m--)
    {
        cin>>a>>b;
        v.pb({b,a});
    }

    sort(v.rbegin(),v.rend());

    int x=0,ans=0;

    for(i=0;i<v.size();i++)
    {
        if((x+v[i].second)<=n)
        {
            ans+=(v[i].first*v[i].second);
            x+=v[i].second;
        }
        else
        {
           int p=n-x;
           ans+=(p*v[i].first);
           break;
        }
    }

    cout<<ans<<endl;

    return 0;
}