import java.util.*;
public class Main{
	public static void main(String[] args){
    	Scanner sc=new Scanner(System.in);
    	int n=sc.nextInt();
    	int t = 1;
    	int can = 0;
    	for(int i=0;i<n;i++) {
    		int m = sc.nextInt();
    		if(m==t) {
    			can++;
    			t++;
    		}
    	}
    	int sum = can==0?-1:n-can;
    	System.out.println(sum);
	}
}
/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