package main

import "fmt"

func main() {
	var B [10]int
	for i := 0; i < 9; i++ {
		fmt.Scan(&B[i])
	}
	for i := 0; i < 10; i++ {
		if B[i] != i+1 {
			fmt.Println(i + 1)
			break
		}
	}
}