Problem:
Score : 400 points

Problem Statement

We have N bricks arranged in a row from left to right.

The i-th brick from the left (1 \leq i \leq N) has an integer a_i written on it.

Among them, you can break at most N-1 bricks of your choice.

Let us say there are K bricks remaining. Snuke will be satisfied if, for each integer i (1 \leq i \leq K), the i-th of those brick from the left has the integer i written on it.

Find the minimum number of bricks you need to break to satisfy Snuke's desire. If his desire is unsatisfiable, print -1 instead.

Constraints

All values in input are integers.

1 \leq N \leq 200000

1 \leq a_i \leq N

Input

Input is given from Standard Input in the following format:

N
a_1 a_2 ... a_N

Output

Print the minimum number of bricks that need to be broken to satisfy Snuke's desire, or print -1 if his desire is unsatisfiable.

Sample Input 1

3
2 1 2

Sample Output 1

1

If we break the leftmost brick, the remaining bricks have integers 1 and 2 written on them from left to right, in which case Snuke will be satisfied.

Sample Input 2

3
2 2 2

Sample Output 2

-1

In this case, there is no way to break some of the bricks to satisfy Snuke's desire.

Sample Input 3

10
3 1 4 1 5 9 2 6 5 3

Sample Output 3

7

Sample Input 4

1
1

Sample Output 4

0

There may be no need to break the bricks at all.

Solution in C# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections.Generic;

namespace D
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string[] A = Console.ReadLine().Split(' ');
            List<int> a = new List<int>();
            int ia = 0;
            int i = 1;
            int j = 1;
            a.Add(0);
            for (j = 0; j < N; j++)
            {
                ia = int.Parse(A[j]);
                a.Add(ia);
            }
            j = 1;i = 1;
            int d = 0;
            while (true)
            {
                if (j < a.Count)
                {
                    if (a[i] == j)
                    {
                        i++; j++;
                        continue;
                    }
                    else
                    {
                        int m = a[i];
                        d++;
                        a.Remove(m);
                    }
                }
                else
                {
                    break;
                }
            }
            if (d == N)
            {
                Console.WriteLine("-1");
            }
            else
            {
                Console.WriteLine(d);
            }
        }
    }
}