#include <iostream>
using namespace std;
int n, k;
long long a[1000000001];

int main() {
	cin >> n >> k;
	long long sum = 0;
	long cnt;
	for (int i = 0; i < n - 1; i++)
		cin >> a[i];
	if (k == 0) {
		for (int i = 0; i < n - 1; i++)
			sum = sum + a[i];
		cout << sum << endl;
		return 0;
	} else {
		for (int i = 0; i < n; i++)
			if (a[i] + k == n) {
				cnt = i;
			}
		for (int i = 0; i < cnt; i++)
			sum = sum + a[i];
		cout << sum << endl;
	}
	return 0;
}