import java.util.*;
 
public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		boolean happy = false;
		int breaking = 0;
		int next = 1;

		for(int i=0; i<N; i++){
			int a = sc.nextInt();
			if(a==next){
				next++;
				happy = true;
			}else{
				breaking++;
			}
		}
		if(happy==true){
			System.out.println(breaking);
		}else{
			System.out.println("-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