package main

import "fmt"

func main() {
	var p int
	a := make([]int, 9)
	for i := 0; i < 9; i++ {
		fmt.Scan(&a[i])
	}
	p = 0
	for _, v := range a {
		if (v - p) != 1 {
			fmt.Println(v - 1)
			return
		}
		p = v
	}
}