Evaluate Mathematical Expression

You are given a string that represents a mathematical expression. The expression contains non-negative integers and the operators +, -, *, and /. Your task is to compute and output the integer result of the expression. The operations should be evaluated following the standard operator precedence rules:

\( \text{Multiplication and Division have higher precedence than Addition and Subtraction.} \)

If the expression is invalid (e.g. contains illegal characters, an incomplete operator sequence, division by zero, or is empty), output None.

Note: When division occurs, the result is converted to an integer by truncating the decimal part (i.e. using the built-in conversion to integer).

inputFormat

The input consists of a single line which is a string representing the mathematical expression. The expression can contain spaces.

outputFormat

Output a single line: the evaluated integer result of the expression or None if the expression is invalid.

3+5*2

13