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.Text;
using System.Collections.Generic;
using System.Linq;
 
class Program {
    static List<long> rep;
    static void Main(string[] args){
        //入力を受け取る
        var N = long.Parse(Console.ReadLine());
        var A = Console.ReadLine().Split().Select(a => long.Parse(a)).ToArray();
         
         var count = 0;
         for(int i = 0;i < N;i++){
             if(A[i] == count+1){
                 count++;
             }
         }
       
         if(count == 0){
             Console.WriteLine(-1);
         }else{
             Console.WriteLine(N-count);
         }
       　
    }
    
    static int LowerBound(long num){
        
        var l = 0;
        var r = rep.Count()-1;
      
        while(l <= r){
            
            var mid = l+(r-l)/2;
            if(rep[mid] < num){
                l = mid+1;
            }else{
               
                r = mid-1;
            }
        }
        return l;
    }     
}