N, K = gets.split(" ").map{|s| s.to_i}
a = gets.split(" ").map{|s| s.to_i}

if a[K-1] == 0 then
	puts 0
else
	lsum = 0
	(K-2).downto(0) {|i|
		lsum += a[i]
		break if a[i] < 2
	}
	rsum = 0
	K.upto(N-1) {|i|
		rsum += a[i]
		break if a[i] < 2
	}
	if a[K-1] == 1 then
		puts [lsum, rsum].max + 1
	else
		puts lsum + rsum + a[K-1]
	end
end