C++ :

#include<iostream>
#include<algorithm>
using namespace std;
int n,m,ans,i,t;
int main()
{
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin>>n>>m;
  pair<int,int> a[m];
  for(i=0;i<m;i++)cin>>a[i].first>>a[i].second;
  sort(a,a+m);
  for(i=0;n>0;i++)t=min(n,a[i].second),n-=t,ans+=t*a[i].first;
  cout<<ans;
  return 0;
}