import (
	"strings"
	"strconv"
)

func calculate(s string) int {
	s = strings.Replace(s, " ", "", -1)
	nums := make([]int, 0)
	ops := make([]byte, 0)
	l, m, n := len(s), 0, 0
	beg := -1
	push := false
	for i := 0; i < l; i++ {
		switch s[i] {
		case '+', '-':
			var num int
			if beg != -1 {
				num = atoi(s, beg, i)
				beg = -1
			} else {
				num = nums[n-1]
				nums = nums[:n-1]
				n--
			}
			if !push && m != 0 { // Pop
				aopb(&nums[n-1], ops[m-1], num)
				ops = ops[:m-1]
				m--
			} else {
				nums = append(nums, num)
				n++
				push = false
			}
			ops = append(ops, s[i])
			m++
		case '(':
			push = true
		case ')':
			var num int
			if beg != -1 {
				num = atoi(s, beg, i)
				beg = -1
			} else {
				num = nums[n-1]
				nums = nums[:n-1]
				n--
			}
			if m != 0 { // Handle expressions like (num)
				aopb(&nums[n-1], ops[m-1], num)
				ops = ops[:m-1]
				m--
			} else {
				nums = append(nums, num)
				n++
			}
		default: // Case '0', '1', ... , '9'
			if beg == -1 {
				beg = i
			}
		}
	}
	if beg != -1 {
		nums = append(nums, atoi(s, beg, l))
	}
	if len(ops) != 0 {
		aopb(&nums[0], ops[0], nums[1])
	}
	return nums[0]
}

func atoi(s string, beg, end int) int {
	i, _ := strconv.Atoi(s[beg:end])
	return i
}

func aopb(a *int, op byte, b int) {
	switch op {
	case '+': *a += b
	case '-': *a -= b
	}
}