import java.util.Scanner;


public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
			
			int N = sc.nextInt();
			int[] a = new int[N]; 
			int check = 1;
			int destroy = 0;
			for(int i=1;i<=N;i++) {
				a[i-1] = sc.nextInt();
				if(a[i-1]==check) {
					check++;
				}else {
					destroy++;
				}
			}
			
			if(destroy==N) {
				System.out.println(-1);
			}else {
				System.out.println(destroy);
			}
			
	}	

}
/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