#include <iostream>
#include <vector>

using lint = long long;

void solve() {
    int n, k;
    std::cin >> n >> k;
    --k;

    std::vector<lint> xs(n);
    for (auto& x : xs) std::cin >> x;

    lint l = 0;
    for (int i = k - 1; i >= 0; --i) {
        l += xs[i];
        if (xs[i] <= 1) break;
    }

    lint r = 0;
    for (int i = k + 1; i < n; ++i) {
        r += xs[i];
        if (xs[i] <= 1) break;
    }

    lint ans;
    if (xs[k] == 0) {
        ans = 0;
    } else if (xs[k] == 1) {
        ans = std::max(l, r) + 1;
    } else {
        ans = l + r + xs[k];
    }

    std::cout << ans << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::cout.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}