//#include <bits/stdc++.h>

#include <vector>

#include <iostream>

#include <queue>

#include <cmath>

#include <map>

#include <cstring>

#include <algorithm>

#include <cstdio>

using namespace std;

#define Riep(n) for(int i=1;i<=n;i++)

#define Riop(n) for(int i=0;i<n;i++)

#define Rjep(n) for(int j=1;j<=n;j++)

#define Rjop(n) for(int j=0;j<n;j++)

#define mst(ss,b) memset(ss,b,sizeof(ss));

typedef long long LL;

template<class T> void read(T&num) {

    char CH; bool F=false;

    for(CH=getchar();CH<''||CH>'';F= CH=='-',CH=getchar());

    for(num=;CH>=''&&CH<='';num=num*+CH-'',CH=getchar());

    F && (num=-num);

}

int stk[], tp;

template<class T> inline void print(T p) {

    if(!p) { puts(""); return; }

    while(p) stk[++ tp] = p%, p/=;

    while(tp) putchar(stk[tp--] + '');

    putchar('\n');

}

const LL mod=1e9+;

const double PI=acos(-1.0);

const LL inf=1e14;

const int N=1e5+;

int n,a,t[];

int main()

{

   read(n);read(a);

   Riep(n)read(t[i]);

   int ans=;

   for(int i=;i<=n;i++)

   {

       if(a-i>&&a+i<=n)

       {

           if(i==)ans+=t[a];

          else if(t[a-i]&&t[a+i])ans+=;

       }

       else if(a-i<&&a+i<=n)

       {

           ans+=t[a+i];

       }

       else if(a-i>=&&a+i>n)ans+=t[a-i];

   }

   cout<<ans<<"\n";

        return ;

}