import java.util.Scanner;
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 count = 0;
		int p = 1;
		for(int i=0;i<N;i++) {
			if(A[i] != p) {
				count++;
			}else {
				p++;
			}
		}
		if(p==1) {
			System.out.println(-1);
		}else {
			System.out.println(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