import java.util.*;
import java.io.*;
public class Main 
{
	public static void main(String[] args) throws IOException{
		BufferedReader br = new BufferedReader(new InputStreamReader (System.in));
		int n = Integer.parseInt(br.readLine());
		String[] arr = br.readLine().split(" ");
		int current_number = 1;
		for (int i = 0; i < n; i++) {
			int current_value = Integer.parseInt(arr[i]);
			if (current_number == current_value) {
				current_number++;
			}
		}
		if (current_number == 1) System.out.println(-1);
		else System.out.println(n - (current_number - 1));
	}
}

/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