import java.util.*;
import java.lang.*;

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