title: Friends Pairing Problem 2
signature: function friendsPairingProblem2($n) {
source_py: def friends_pairing_problem_2(n):
      (a, b, c) = (1, 2, 0)
      if (n <= 2):
          return n
      for i in range(3, (n + 1)):
          c = (b + ((i - 1) * a))
          a = b
          b = c
      return c
source_cpp: int friends_pairing_problem_2(int n) {
    int a = 1, b = 2, c = 0;
    if ( n <= 2 ) {
      return n;
    }
    for ( int i = 3;
    i <= n;
    i ++ ) {
      c = b + ( i - 1 ) * a;
      a = b;
      b = c;
    }
    return c;
  }