import java.util.*;
 
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt(); //レンガの数
        int count = 0;
        int  a = 0;
        for(int i = 1; i <= N; i++) {
            a = sc.nextInt();
            if(a == count+1) {
                count++;
            }
        }
        
        if(count == 0) {
            System.out.println(-1);
        }else {
            System.out.println(N-count);
        }
    }
}
/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