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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int main(void){
    ll n;
    ll a[200001];
    ll b[200001]; //砕いた後のレンガの並び
    cin >> n;
    
    
    for(int i=0; i<n; i++){
        cin >> a[i];
        //if (a[i] == i+1) b[i] = i+1;
    }
    
    int ans = 0;
     
    /* 
    for(int i=0; i<n; i++){
        if (b[i] != i+1) break;
    }
    */
    ll bricks = 0;//砕くレンガの数
    ll tmp = 1;
    for(int i=0; i<n; i++){
        if(a[i] == tmp) {
            tmp++;
        }else if(a[i] != tmp){
            bricks++;
        }
    }   
    
    if (bricks == n) bricks = -1;
   
    std::cout << bricks << std::endl;

    return 0;
}