
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

	public static void main(String[] args) throws NumberFormatException, IOException {
		// TODO Auto-generated method stub
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		
		int n = Integer.parseInt(br.readLine());
		String[] tmpArray = br.readLine().split(" ");
		
		int input[] = new int[n];
		
		for(int i = 0; i < n; i++) {
			input[i] = Integer.parseInt(tmpArray[i]);
		}
		
		int target = 1;
		int length = 0;
		for(int i = 0; i < n; i++) {
			if(input[i] == target) {
				target++;
				length++;
			}
		}
		
		System.out.println(length > 0 ? n - length : -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