import java.util.ArrayList;
import java.util.Scanner;

 
public class Main {
 
	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		ArrayList<Integer> brick = new ArrayList<>();
		int ans = 0;
		for(int i = 0; i < n; i++) {
			brick.add(sc.nextInt());
		}
		int index = 1; 
		
		for(int i = 1; i <= n; i++) {
			if(brick.get(i-1) == index) {
				index++;
			}else {
				ans++;
			}
		}
		
		System.out.println(index == 1 ? -1 : ans);
		sc.close();
	}
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz