package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

func main() {
	var input = bufio.NewScanner(os.Stdin)
	var output int
	var digits []int
	for input.Scan() {
		line := input.Text()

		digits = []int{}
		values := strings.Split(line, " ")

		for _, v := range values {
			d, _ := strconv.Atoi(v)
			digits = append(digits, d)
		}

		output = missingNumber(digits)
		fmt.Println(output)
	}
}

func missingNumber(nums []int) int {
	var length = len(nums)
	if length-1 == nums[length-1] {
		return length
	}
	var left, right = 0, len(nums) - 1
	var mid int
	for left < right {
		mid = (left + right) / 2
		if nums[mid] > mid {
			right = mid
		} else {
			left = mid + 1
		}
	}

	return left
}