#include <iostream>
using namespace std;

int main ( void )
{
  int a, b, c, k;
  ios::sync_with_stdio ( false );

  cin >> a >> b >> k >> c;
  if ( k == 0 )
  {
    cout << 0 << endl;
    return 0;
  }
  if ( c != a && c != b )
  {
    cout << 0 << endl;
    return 0;
  }
  if ( a == b )
  {
    cout << k << endl;
    return 0;
  }

  int res = k, comb = k;
  for ( int i = 2; i <= k; ++ i )
  {
    comb *= (k - i + 1);
    comb /= (i-1);
    res += comb;
  }

  cout << res << endl;

  return 0;
}