Need help with this python script

[astonmartin]

OK I pretty new to python but I'm pretty sure this should work. everything is defined, only posted whats causing the problem. expression is a Reverse Polish Notation for a calculator (ex 1 2 +) and has already been parsed at this point into single array elements. All I get after running is a SERVER ERROR.

stack = []
i = 0
j = 0

for i in range(len(expression))
	if len(expression) <= 2
		print "Invalid Expression: Not enough elements"
		break
	if expression[i] == '+' || expression[i] == '-' || expression[i] == '*'
		if i == 0 || i == 1
			print "Invalid Expression: Operator out of place!"
			break
		if expression[i] == '+'
			stack[j-2] = stack[j-2] + stack[j-1]
			j = j - 1
			stack[j] = ""
		if expression[i] == '-'
			stack[j-2] = stack[j-2] - stack[j-1]
			j = j - 1
			stack[j] = ""
		if expression[i] == '*'
			stack[j-2] = stack[j-2] * stack[j-1]
			j = j - 1
			stack[j] = ""
	else
		stack[j] = int(expression[i])
		j = j + 1

print stack[0]