text: # https://www.acmicpc.net/problem/17828
  
  if __name__ == '__main__':
      input = __import__('sys').stdin.readline
      N, X = map(int, input().split())
      res = __import__('collections').deque()
  
      if N * 26 < X or N > X:
          print('!')
          exit()
  
      for As in range(N - 1, -1, -1):
          value = min(X - As, 26)
          res.appendleft(value)
          X -= value
  
      print(''.join(map(lambda x: chr(x + 64), res)))
  