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 Go (1.6) (judge verdict: Runtime Error)

package main

import "fmt"

func main(){
	var N int
	var a[200000] int
	var b[200000] int
	var K, count int

	fmt.Scan(&N)

	for i := 1 ; i <= N ; i++ {
		fmt.Scan(&a[i])
	}
	count = 1
	for i := 1 ; i <= N ; i++ {
		if count == a[i]{
			b[i] = a[i]
			count++
		}else {
			K++
		}
		// fmt.Printf("%d ", a[i])
	}
//	fmt.Println()

	if K == 0 {
		fmt.Println("0")
	}else if K == N {
		fmt.Println("-1")
	}else {
		fmt.Println(K)
	}
}