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

public class Main {

	public static void main(String[] args) throws Exception {
		// TODO 自動生成されたメソッド・スタブ
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		String W = in.readLine();

		int N = Integer.parseInt(W);
		W = in.readLine();

		String[] Ws = W.split(" ");

		int[] list = new int[N];
		for(int i=0;i<N;i++){
			list[i] = Integer.parseInt(Ws[i]);
		}

		int i=1;
		int ans = 0;

		for(int k=0;k<N;k++){
			if(list[k]==i){
				i++;
			}else{
				ans++;
			}
		}
		if(i==1){
			ans = -1;
		}
		System.out.println(ans);



	}
}

/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