# Dumb parser for '+' and '-'

OP = '+-'

stack = []
tree = []
state = 1


def parse():
    global s, state, stack, tree
    x, s = s[0], s[1:]
    if x in OP:
        operator = 1
    else:
        operator = 0
    if state == 1:
        if not operator:
            stack.append(x)
            state = 2
            return
        else:
            raise Exception, 'Parse Error'
    elif state == 2:
        if operator:
            stack.append(x)
            state = 3
            return
        else:
            raise Exception, 'Parse Error'
    elif state == 3:
        if not operator:
            op = stack.pop()
            var = stack.pop()
            tree = [var, op, x]
            state = 4
            return
        else:
            raise Exception, 'Parse Error'
    elif state == 4:
        if x == '$':
            return
        if operator:
            stack.append(tree)
            stack.append(x)
            state = 3
            return
    else:
        raise Exception, 'Parse Error'

if __name__ == '__main__':
    global s
    s = 'a+b+c+d+e$'
    while len(s) > 1:
        parse()
    print tree