def verify_sin(sin):
    '''verify_sin(str) - > boolean
     
     Given a string argument returns True if the string entered represents 
     a valid sin number. Otherwise returns False. Also returns False if the
     sin entered is not a string or if it is of the wrong length.'''
    
    if ( type(sin) != str ):
        return False
    
    sin.replace('-','')
    
    if ( len(sin) != 9 ):
        return False
    
    count = 0
    sum1 = 0
    sum2 = 0
   
    
    for digit in sin:
                        
        if count % 2 == 0 and count < 9:
            sum1 += int(digit)
            count += 1
        if count % 2 != 0 and count < 9 and int(digit) * 2 < 10:
            sum2 = sum2 + (int(digit) * 2)
            count += 1
        if count % 2 != 0 and count < 9 and int(digit) * 2 >= 10:
            sum2 = sum2 + int(digit) * 2 / 10 + int(digit) * 2 % 10
            count += 1
    sum_final = 10 - ( ( sum1 + sum2 ) % 10 )
    return str(sum_final) == sin[10]