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# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;
class Program
{
	static void Main(string[] args)
	{
	    int n = IO.GetInt();
	    List<int> a = IO.GetIntList();
	    int count = 0;
	    int target = 1;
	    for(int i=0;i<n;i++){
	        if(a[i]==target){
	            target++;
	        }else{
	            count++;
	        }
	    }
	    Console.WriteLine((count!=n)?count:-1);
	}
    
}

static class IO
{
    static public string GetString() { return Console.ReadLine(); }
    static public int GetInt() { return int.Parse(Console.ReadLine()); }
    static public long GetLong() { return long.Parse(Console.ReadLine()); }
    static public List<string> GetStringList() { return Console.ReadLine().Split(' ').ToList(); }
    static public List<int> GetIntList() { return Console.ReadLine().Split(' ').Select(int.Parse).ToList(); }
    static public List<long> GetLongList() { return Console.ReadLine().Split(' ').Select(long.Parse).ToList(); }
    static public List<double> GetDoubleList() { return Console.ReadLine().Split(' ').Select(double.Parse).ToList(); }
    static public void WriteStrings<T>(IEnumerable<T> values) { foreach (var o in values) { Console.Write(o + " "); } }
}