3749. Evaluate Valid Expressions

You are given a string expression that represents a nested mathematical expression in a simplified form.

A valid expression is either an integer literal or follows the format op(a,b), where:

• op is one of "add", "sub", "mul", or "div".
• a and b are each valid expressions.

The operations are defined as follows:

• add(a,b) = a + b
• sub(a,b) = a - b
• mul(a,b) = a * b
• div(a,b) = a / b

Return an integer representing the result after fully evaluating the expression.

Example 1:

Example 2:

Example 3:

Constraints:

• 1 <= expression.length <= 105
• expression is valid and consists of digits, commas, parentheses, the minus sign '-', and the lowercase strings "add", "sub", "mul", "div".
• All intermediate results fit within the range of a long integer.
• All divisions result in integer values.