
// @snippet<sh19910711/contest:headers.cpp>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
#include <numeric>
#include <limits>
#include <complex>
#include <functional>
#include <iterator>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>

// @snippet<sh19910711/contest:solution/define_classes.cpp>
namespace solution {
  class Solution;
  class Solver;
  class Storages;
}

// @snippet<sh19910711/contest:solution/interface.cpp>
namespace solution {
  class SolutionInterface {
  public:
    virtual ~SolutionInterface() {};
    virtual int run() = 0;
    
  protected:
    SolutionInterface() {}
    virtual void pre_calc() {}
    virtual bool action() = 0;
    virtual void init() {};
    virtual bool input( Storages& s ) { return false; };
    virtual void output( const Storages& s ) const {};
    virtual void before_action( const int& test_no ) const {}
    virtual void after_action( const int& test_no ) const {}
  };
}

// @snippet<sh19910711/contest:solution/solution-base.cpp>
namespace solution {
  class SolutionBase: public SolutionInterface {
  public:
    virtual ~SolutionBase() {}
    virtual int run() {
      this->pre_calc();
      for (;;) {
        this->before_action(-1);
        if ( ! this->action() ) { this->after_action(-1); break; }
        this->after_action(-1);
      };
      return 0;
    }
  protected:
    SolutionBase() {}
  };
}

namespace math {
  template <int SIZE> class PrimeChecker {
  public:
    void init() {
      std::fill( P + 0, P + SIZE, false );
      for ( int i = 3; i < SIZE; i += 2 )
        P[i] = true;
      P[2] = true;
      for ( int i = 3; i < SIZE; i += 2 ) {
        if ( P[i] )
          for ( int j = i + i; j < SIZE; j += i )
            P[j] = false;
      }
    }
    bool P[SIZE];
  };
}

// @snippet<sh19910711/contest:solution/typedef.cpp>
namespace solution {
  typedef std::istringstream ISS;
  typedef std::ostringstream OSS;
  typedef long long Int;
  typedef long double Double;
  typedef std::string String;
  
}

// @snippet<sh19910711/contest:solution/namespace-area.cpp>
namespace solution {
  // namespaces, types
  using namespace std;
  const int SIZE = 1000000 + 11;
  typedef math::PrimeChecker<SIZE> PrimeChecker;
}

// @snippet<sh19910711/contest:solution/consts-area.cpp>
namespace solution {
  // constant vars
  
  
}

// @snippet<sh19910711/contest:solution/storages-area.cpp>
namespace solution {
  // storages
  class Storages {
  public:
    Int N;
    PrimeChecker P;
    Int result;
  };
  
  Storages global_storages;
}

// @snippet<sh19910711/contest:solution/solver-area.cpp>
namespace solution {
  class Solver {
  public:
    void solve( Storages& s ) {
      s.result = count_numbers(s.N, s.P);
    }
    
    static Int count_numbers( const Int& N, const PrimeChecker& P ) {
      Int res = 0;
      for ( int i = 0; i * 2 <= N; ++ i ) {
        if ( P.P[i] && P.P[N - i] )
          res ++;
      }
      return res;
    }
    
  protected:
    
  };
}

// @snippet<sh19910711/contest:solution/solution.cpp>
namespace solution {
  class Solution: public SolutionBase {
  public:
    Solution() {}
    Solution(Storages* p): storages(p) {
      p->P.init();
    }
    
  protected:
    virtual bool action() {
      this->init();
      if ( ! this->input(*this->storages) )
        return false;
      solver.solve(*this->storages);
      this->output(*this->storages);
      return true;
    }
    
    bool input( Storages& s ) {
      return std::cin >> s.N && s.N;
    }
    
    void output( const Storages& s ) const {
      std::cout << s.result << std::endl;
    }
    
  private:
    Solver solver;
    Storages* storages;
  };
}

// @snippet<sh19910711/contest:main.cpp>
#ifndef __MY_UNIT_TEST__
int main() {
  std::cin.sync_with_stdio(false);
  return solution::Solution(&solution::global_storages).run();
}
#endif