import std.stdio, std.string, std.conv, std.range; import std.algorithm, std.array, std.typecons, std.container; import std.math, std.numeric, std.random, core.bitop; enum inf = 1_001_001_001; enum infl = 1_001_001_001_001_001_001L; enum mod = 1_000_000_007L; struct Mint { long x; this(long a) { x = a % mod; if (x < 0) x += mod; } this(Mint a) { x = a.x; } ref Mint opAssign(long a) { this = Mint(a); return this; } ref Mint opOpAssign(string op)(Mint rhs) { static if (op == "+") { x += rhs.x; if (x >= mod) x -= mod; } static if (op == "-") { x -= rhs.x; if (x < 0) x += mod; } static if (op == "*") { (x *= rhs.x) %= mod; } static if (op == "/") { this *= rhs.inv(); } return this; } ref Mint opOpAssign(string op)(long rhs) { static if (op == "^^") { this = powmod(this, rhs); return this; } else { return mixin("this " ~ op ~ "= Mint(rhs)"); } } const Mint powmod(Mint a, long b) { Mint res = 1, p = a; while (b > 0) { if (b & 1) res *= p; p *= p; b /= 2; } return res; } const Mint inv() { return powmod(this, mod - 2); } Mint opUnary(string op)() if (s == "-") { return Mint(-x); } Mint opBinary(string op, T)(const T rhs) { return mixin("Mint(this) " ~ op ~ "= rhs"); } Mint opBinaryRight(string op)(const long rhs) { return mixin("Mint(rhs) " ~ op ~ "= this"); } bool opEquals(Mint a, Mint b) { return a.x == b.x; } bool opEquals(long rhs) { long y = rhs % mod; if (y < 0) y += mod; return x == y; } string toString() { import std.conv : to; return x.to!string; } } unittest { long powmod(long a, long b) { return b > 0 ? powmod(a, b / 2)^^2 % mod * a^^(b & 1) % mod : 1L; } auto a = Mint(2), b = Mint(3); assert(a + b == 5); assert(a + 5 == 7); assert(1 + b == 4); assert(a - b == mod - 1); assert(a * b == 6); assert(a / b == 2 * powmod(3, mod - 2)); assert(a^^10 == 1024); Mint z; assert(z == 0); (z += 2) *= 3; assert(z == 6); } void main() { int N, K; scan(N, K); auto a = readln.split.to!(long[]); a = [0L] ~ a ~ [0L]; N += 2; if (a[K] == 0) { writeln(0); return; } auto as = new long[](N + 1); foreach (i ; 1 .. N + 1) { as[i] += as[i - 1] + a[i - 1]; } int l = K, r = K; while (a[l] > 1) { l--; } while (a[r] > 1) { r++; } r++; auto ans = as[r] - as[l]; writeln(ans); } void scan(T...)(ref T args) { auto line = readln.split; foreach (ref arg; args) { arg = line.front.to!(typeof(arg)); line.popFront; } assert(line.empty); } void fillAll(R, T)(ref R arr, T value) { static if (is(typeof(arr[] = value))) { arr[] = value; } else { foreach (ref e; arr) { fillAll(e, value); } } } bool chmin(T, U...)(ref T x, U args) { bool isChanged; foreach (arg; args) if (x > arg) { x = arg; isChanged = true; } return isChanged; } bool chmax(T, U...)(ref T x, U args) { bool isChanged; foreach (arg; args) if (x < arg) { x = arg; isChanged = true; } return isChanged; } void yes(bool ok, string y = "Yes", string n = "No") { return writeln(ok ? y : n); }