Problem:
Score : 500 points

Problem Statement

Given are a sequence of N positive integers A_1, A_2, \ldots, A_N, and a positive integer K.

Find the number of non-empty contiguous subsequences in A such that the remainder when dividing the sum of its elements by K is equal to the number of its elements. We consider two subsequences different if they are taken from different positions, even if they are equal sequences.

Constraints

All values in input are integers.

1 \leq N \leq 2\times 10^5

1 \leq K \leq 10^9

1 \leq A_i \leq 10^9

Input

Input is given from Standard Input in the following format:

N K
A_1 A_2 \cdots A_N

Output

Print the number of subsequences that satisfy the condition.

Sample Input 1

5 4
1 4 2 3 5

Sample Output 1

4

Four sequences satisfy the condition: (1), (4,2), (1,4,2), and (5).

Sample Input 2

8 4
4 2 4 2 4 2 4 2

Sample Output 2

7

(4,2) is counted four times, and (2,4) is counted three times.

Sample Input 3

10 7
14 15 92 65 35 89 79 32 38 46

Sample Output 3

8

Solution in Rust (1.15.1) (judge verdict: Accepted)

use std::fmt::Display;
use std::io::{stdin, Read};
use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
use std::str::FromStr;

#[derive(Clone, Copy, PartialEq, Debug, Default)]
pub struct Mint<T>(pub T, pub T);

impl Mint<usize> {
    pub fn pow(self, exp: usize) -> Self {
        let mut exp = exp;
        let mut v = self;
        let mut ret = Mint(1, self.1);
        while exp > 0 {
            if exp & 1 == 1 {
                ret *= v;
            }
            v *= v;
            exp >>= 1;
        }
        ret
    }
    pub fn inv(self) -> Self {
        self.pow(self.1 - 2)
    }
}

impl<T: Display + Copy + PartialEq> Display for Mint<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}

impl Add<usize> for Mint<usize> {
    type Output = Self;
    fn add(self, rhs: usize) -> Self::Output {
        Mint((self.0 + rhs) % self.1, self.1)
    }
}

impl Add for Mint<usize> {
    type Output = Self;
    fn add(self, rhs: Self) -> Self::Output {
        self + rhs.0
    }
}

impl AddAssign<usize> for Mint<usize> {
    fn add_assign(&mut self, rhs: usize) {
        *self = *self + rhs;
    }
}

impl AddAssign for Mint<usize> {
    fn add_assign(&mut self, rhs: Self) {
        *self = *self + rhs;
    }
}

impl Sub<usize> for Mint<usize> {
    type Output = Self;
    fn sub(self, rhs: usize) -> Self::Output {
        let rhs = rhs % self.1;
        let lhs = self.0 + if self.0 < rhs { self.1 } else { 0 };
        Mint(lhs - rhs, self.1)
    }
}

impl Sub for Mint<usize> {
    type Output = Self;
    fn sub(self, rhs: Self) -> Self::Output {
        self - rhs.0
    }
}

impl SubAssign<usize> for Mint<usize> {
    fn sub_assign(&mut self, rhs: usize) {
        *self = *self - rhs
    }
}

impl SubAssign for Mint<usize> {
    fn sub_assign(&mut self, rhs: Self) {
        *self = *self - rhs
    }
}

impl Mul<usize> for Mint<usize> {
    type Output = Self;
    fn mul(self, rhs: usize) -> Self::Output {
        Mint((self.0 * rhs) % self.1, self.1)
    }
}

impl Mul for Mint<usize> {
    type Output = Self;
    fn mul(self, rhs: Self) -> Self::Output {
        self * rhs.0
    }
}

impl MulAssign<usize> for Mint<usize> {
    fn mul_assign(&mut self, rhs: usize) {
        *self = *self * rhs;
    }
}

impl MulAssign for Mint<usize> {
    fn mul_assign(&mut self, rhs: Self) {
        *self = *self * rhs;
    }
}

impl Div<usize> for Mint<usize> {
    type Output = Self;
    fn div(self, rhs: usize) -> Self::Output {
        self * Mint(rhs, self.1).pow(self.1 - 2)
    }
}

impl Div for Mint<usize> {
    type Output = Self;
    fn div(self, rhs: Self) -> Self::Output {
        self / rhs.0
    }
}

impl DivAssign<usize> for Mint<usize> {
    fn div_assign(&mut self, rhs: usize) {
        *self = *self / rhs;
    }
}
impl DivAssign for Mint<usize> {
    fn div_assign(&mut self, rhs: Self) {
        *self = *self / rhs;
    }
}
macro_rules! parse {
    ($it: ident) => {
        $it.next().unwrap().parse().unwrap()
    };
    ($it: ident,2) => {
        (parse!($it), parse!($it))
    };
    ($it: ident,3) => {
        (parse!($it), parse!($it), parse!($it))
    };
    ($it: ident,4) => {
        (parse!($it), parse!($it), parse!($it), parse!($it))
    };
    ($it: ident, $n: expr) => {
        (&mut $it).take($n).map(|e| e.parse().unwrap()).collect();
    };
}

fn solve(s: &str) {
    let mut it = s.split_whitespace();
    let (n, k): (usize, usize) = parse!(it, 2);
    let a: Vec<_> = (0..n).map(|_| Mint::<usize>(parse!(it), k)).collect();
    let mut sma = std::collections::HashMap::<usize, usize>::new();
    let mut s = Mint(0, k);
    *sma.entry(1).or_insert(0) += 1;
    let mut q = std::collections::VecDeque::new();
    q.push_back(1);
    let mut ret = 0;
    for i in 0..n {
        s += a[i];
        if q.len() >= k {
            let si = q.pop_front().unwrap();
            *sma.entry(si).or_insert(1) -= 1;
        }
        let si = (s - i).0;
        let e = sma.entry(si).or_insert(0);
        ret += *e;
        *e += 1;
        q.push_back(si);
    }
    println!("{}", ret);
}

fn main() {
    let mut s = String::new();
    stdin().read_to_string(&mut s).unwrap();
    solve(&s);
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_input() {
        let s = "
        ";
        solve(s);
    }
}