package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	in := bufio.NewReader(os.Stdin)
	out := bufio.NewWriter(os.Stdout)
	defer out.Flush()

	var n, a int
	if _, err := fmt.Fscan(in, &n, &a); err != nil {
		return
	}
	t := make([]int, n+1)
	for i := 1; i <= n; i++ {
		fmt.Fscan(in, &t[i])
	}

	maxd := a - 1
	if n-a > maxd {
		maxd = n - a
	}

	ans := 0
	for d := 0; d <= maxd; d++ {
		l, r := a-d, a+d
		if l >= 1 && r <= n {
			if l == r {
				ans += t[l]
			} else if t[l] == 1 && t[r] == 1 {
				ans += 2
			}
		} else if l >= 1 && r > n {
			ans += t[l]
		} else if l < 1 && r <= n {
			ans += t[r]
		}
	}

	fmt.Fprintln(out, ans)
}