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 Rust (1.15.1) (judge verdict: Accepted)

macro_rules! extract {
    (String, $iter:expr) => {
        $iter.next().unwrap().trim().to_owned()
    };
    ($t:tt, $iter:expr) => {
        $iter.next().unwrap().trim().parse::<$t>().unwrap()
    };
}

macro_rules! read_split_impl {
    ( $e:expr, $( $t:tt ),* ) => {{
        let mut s = String::new();
        $e.read_line(&mut s).unwrap();
        let s = s.trim_right();
        let mut iter = s.split_whitespace();
        ( $(extract!($t, iter)),* )
    }};
}

macro_rules! read_split {
    ( $( $t:tt ),* ) => {
        read_split_impl!(std::io::stdin(), $($t),* )
    };
}

macro_rules! read_vec_impl {
    ($input:expr, $t:tt, $size: expr) => {
        {
            let mut s = String::new();
            $input.read_line(&mut s).unwrap();
            let s = s.trim_right();
            let mut v = Vec::with_capacity($size);
            for x in s.split_whitespace() {
                v.push(x.parse::<$t>().unwrap());
            }
            v
        }
    };
}

macro_rules! read_vec {
    ($t: tt, $size: expr) => {
        read_vec_impl!(std::io::stdin(), $t, $size)
    };
}

fn solve(v: &[i64]) -> i64 {
    let i = v.iter().position(|x| { *x == 1 });
    if let Some(i) = i {
        let mut result = i;
        let mut next = 2;
        for x in &v[i + 1 ..] {
            if *x == next {
                next += 1;
            } else {
                result += 1;
            }
        }
        result as i64
    } else {
        -1
    }
}

fn main() {
    let n = read_split!(usize);
    let v = read_vec!(i64, n);

    print!("{}", solve(&v));
}