#include <iostream>
using namespace std;
 
unsigned CountBefore(unsigned N) { return (N-1)/3+1; }
unsigned CountBetween(unsigned A,unsigned B) { return CountBefore(B)-(A?CountBefore(A-1):0); }
 
int main()
  {
   cout<<CountBetween(1,20)<<endl;
   cout<<CountBetween(21,1000)<<endl;
   cout<<CountBetween(1001,1000000)<<endl;
   cout<<CountBetween(1000001,10000000)<<endl;
   cin.get(); // jeżeli tego potrzebujesz to zmień IDE
   return 0;
  }

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