<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def is_amex(n):
 """Checks if credit card number fits the american express format."""

    n, length = str(n), len(str(n))

    if length == 15:
        if n[0] == '3' and (n[1] == '4' or n[1] == '7'):
            return True
    return False