package main

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

func main() {
	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 = findRepeatNumber(digits)
		fmt.Println(output)
	}
}

func findRepeatNumber(nums []int) int {
	var record = map[int]bool{}
	for _, v := range nums {
		if _, ok := record[v]; !ok {
			record[v] = true
			continue
		}
		return v
	}

	return 0
}