import sys

'''
    expressions.py
    Jeff Ondich, 19 May 2010

    This program implements and computes values for infix arithmetic
    expressions involving integers and the plus-sign.  It does so
    by using functions that mimic the productions of this grammar:

        <expression> -> <number> | <number>+<expression>
        <number> -> 0|1|2|3|4|5|6|7|8|9

    where <expression> is the start symbol and <number> is a single-digit
    non-negative integer.

    Each function returns the value of the expression or number that
    it finds at the beginning of the string passed to it, combined with
    the string left over after that number or expression has been consumed.
    For example, expression('2+3') returns (5, ''), while number('4+5')
    returns (4, '+5').
'''

def expression(s):
    (value, leftover) = number(s)
    if len(leftover) > 0:
        if leftover[0] == '+':
            (tmpValue, leftover) = expression(leftover[1:])
            value += tmpValue
        else:
            raise Exception('Invalid expression')
    return (value, leftover)

def number(s):
    if len(s) == 0 or not s[0].isdigit():
        raise Exception('Invalid numerical constant')

    return (ord(s[0]) - ord('0'), s[1:])

if __name__ == '__main__':
    if len(sys.argv) != 2:
        sys.stderr.write('Usage: %s arithmetic-expression\n' % (sys.argv[0]))
        exit()

    arithmeticExpression = sys.argv[1]
    try:
        (value, leftover) = expression(arithmeticExpression)
    except Exception, e:
        print e
        exit()

    if len(leftover) > 0:
        print '"%s" is an invalid expression' % (arithmeticExpression)
    else:
        print 'The value of %s is %d' % (arithmeticExpression, value)