def solve(s):
    """You are given a string s.
    if s[i] is a letter, reverse its case from lower to upper or vise versa, 
    otherwise keep it as it is.
    If the string contains no letters, reverse the string.
    The function should return the resulted string.
    Examples
    solve("1234") = "4321"
    solve("ab") = "AB"
    solve("#a@C") = "#A@c"
    
    Example solution:
    # line 1
    flg = 0
    # line 2
    idx = 0
    # line 3
    new_str = list(s)
    # line 4
    for i in s:
        # line 5
        if i.isalpha():
            # line 6
            new_str[idx] = i
            # line 7
            flg = 1
        # line 8
        idx += 1
    # line 9
    s = ""
    # line 10
    for i in new_str:
        # line 11
        s += i
    # line 12
    if flg == 0:
        # line 13
        return s[len(s)::-1]
    # line 14
    return s
    
    """
    # Please print out which line of the above program contains an error. E.g. if the bug is on line 4 then print 4
    # END OF CONTEXT
    print("6")
    # END OF SOLUTION

def check(candidate):

    import io
    from contextlib import redirect_stdout

    f = io.StringIO()
    with redirect_stdout(f):
        candidate([])
    out = f.getvalue().strip('\n')

    assert "6" == out
    for i in range(0, 10):
        if i != 6:
            assert str(i) != out

if __name__ == '__main__':
    check(solve)
