Evaluating a bracketed expression

A formula is defined as follows:

• a single digit (0–9) is a formula;
• if $A$ and $B$ are formulas, then (A+B), (A-B) and (A*B) are formulas too.

For example, (2+(3*4)) and ((9-7)*(1-6)) are formulas. Compute the value of the given formula.

A single line with the formula, without spaces, of length up to $1000$. All intermediate results do not exceed $10^{18}$ in absolute value.

A single integer — the value of the formula.

The definition of a formula is recursive, and so is the evaluation. The function reads one formula starting at the current character: if it is a digit, return it; if it is (, read the left formula (calling itself), the sign and the right formula, skip ) and return the result. Keeping the current position in a global variable is convenient.

Second example: $(9 - 7) \cdot (1 - 6) = 2 \cdot (-5) = -10$. A 64-bit type is needed.