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();
		int N_=0;
		ArrayList<Integer> a = new ArrayList<Integer>();
		for (int i = 0; i < N; i++) {
			a.add(sc.nextInt());
		}

		if(a.indexOf(1)==-1){
			System.out.println(-1);
			return;
		}
		for(int i = 0;i<N;i++){
			if(a.get(i) == N_+1){
				N_++;
			}
		}
		sc.close();
		System.out.println(N-N_);
	}
}

/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