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 (GCC 5.4.1) (judge verdict: Accepted)

#include<stdio.h>

int breakblocks(int a[], int N, int i, int x)
{
  int j=x;
  while(a[j-1]!=i && j<=N){j++;}
  return j;
}

int main()
{
  int N;
  scanf("%d",&N);
  int a[200001];
  for(int i=0;i<N;i++){
    scanf("%d",&a[i]);
  }
  
  int count=0,temp=1;
  
  for(int i=1;i<=N;i++)
  {
    temp = breakblocks(a,N,i,temp);
    if(temp>=N+1){
      if(i==1){count=-1; break;}
      if(i!=1){count=N-i+1; break;}
    }
    count = 0;
  }
  
  printf("%d",count);
  
}