
import java.util.*;

public class Main {
    public static void main(String[] args) {

        //solve_148_A();
        //solve_148_B();
        //solve_148_C();
        solve_148_D();
        return;
    }

    private static void solve_148_D() {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] A = new int[N+1];
        int counter = 0;

        for(int i=1; i<=N; i++){
            A[i] = sc.nextInt();
        }

        for(int i=1; i<=N; i++){
            if(A[i] == (counter+1)){
                counter++;
            }else{
                //  Nothing
            }
        }
        if(counter == 0){
            counter = -1;
        }else{
            counter = N - counter;
        }
        System.out.println(counter);
    }
}
/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