use std::cmp::min;

fn comb(n: usize, r: usize) -> usize {
    if n == 0 { return 0; }
    if r == 0 { return 1; }
    let select = min(r, n - r);
    let mut result = 1usize;
    (n-select+1..=n).for_each(|i| { result *= i; });
    (1..=select).for_each(|i| { result /= i; });
    result
}

fn solve(n: usize, k: usize) -> usize {
    let uppers = n - k;
    let mut result: usize = 0;
    if n == 1 { return 0; }
    if k == n {
        result = 1;
    } else if k == 1 {
        (1..n).for_each(|i| {
            result += solve(n-1, i);
        });
    } else {
        result += (0..=uppers).map(|i| comb(uppers, i)).sum::<usize>();
    }
    result
}

fn main() {
    let mut nk = String::new();
    std::io::stdin().read_line(&mut nk).ok();
    let nk: Vec<usize> = nk.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let n = nk[0];
    let k = nk[1];

    println!("{}", solve(n, k));
}