import java.util.*;
import java.util.Scanner;
class Main {
	
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        Queue<Integer> queue = new ArrayDeque<>();
        for(int i=0; i<A; i++) {
        		int a = sc.nextInt();
        		queue.add(a);
        }
        int j = 1;
        for(int i=0; i<A; i++) {
        		int a = queue.poll();
        		if(a == j) {
        			j++;
        		}
        }
        if(j == 1) {
        		System.out.println(-1);
        } else {
        		System.out.println(A-j+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