MOD = 10 ** 9 + 7
INF = 10 ** 10
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)

def main():
    n,k = map(int,input().split())
    a = list(map(int,input().split()))
    k -= 1

    if a[k] == 0:
        print(0)
        return 
    
    if a[k] == 1:
        tmp1 = 0
        for i in range(k + 1,n):
            if a[i] > 1:
                tmp1 += a[i]
            else:
                tmp1 += 1
                break
        
        tmp2 = 0
        for i in range(k - 1,-1,-1):
            if a[i] > 1:
                tmp2 += a[i]
            else:
                tmp2 += 1
                break
        
        ans = max(tmp1,tmp2) + 1
        print(ans)
        return
    
    if a[k] > 1:
        tmp1 = 0
        for i in range(k + 1,n):
            if a[i] > 1:
                tmp1 += a[i]
            else:
                tmp1 += 1
                break
        
        tmp2 = 0
        for i in range(k - 1,-1,-1):
            if a[i] > 1:
                tmp2 += a[i]
            else:
                tmp2 += 1
                break
        
        ans = tmp1 + tmp2 + a[k]
        print(ans)
        return
            
if __name__ =='__main__':
    main()