Evaluate Reverse Polish Notation Expression

Given an arithmetic expression in Reverse Polish Notation (RPN), evaluate the result using a stack-based algorithm.

The input is a single line string containing tokens separated by spaces. Each token is either an integer or one of the operators +, -, *, and /. The division operation should truncate toward zero (i.e., perform integer division). Formally, if \(a\) and \(b\) are integers and \(b \neq 0\), then $$ \frac{a}{b}=\operatorname{trunc}\left(\frac{a}{b}\right). $$

It is guaranteed that the given RPN expression is valid and will evaluate to a single integer result.

inputFormat

The input consists of a single line containing a valid RPN expression, where tokens are separated by spaces.

Example: 2 1 + 3 *

outputFormat

Output a single integer which is the result of evaluating the RPN expression.

## sample

2 1 + 3 *

9

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