#include <iostream>
#include <deque>
using namespace std;

struct elmt
{
	int d_time, value;
};

void fillDeque(deque<elmt> &d, int k);
void push_val(deque<elmt> &d, int k, int i);
int makeStep(deque<elmt> &d, int k, int offset);

int main()
{
	freopen("deque.in", "r", stdin);
	freopen("deque.out", "w", stdout);

	int n, k, offset;
	long long sum = 0;
	deque<elmt> deq;

	cin >> n >> k;
	fillDeque(deq, k);
	offset = k-1;


	while (offset < n)
	{
		sum += makeStep(deq, k, offset);
		offset++;
	}
	cout << sum;
}

void fillDeque(deque<elmt> &d, int k)
{
	for (int i = 0; i < k-1; i++)
	{
		//push_val(d, k, i);
		int val;
		elmt e;
		cin >> val;
		if (d.empty())
		{
			e.value = val;
			e.d_time = i + k;
			d.push_back(e);
		}
		else
		{
			while (!d.empty() && d.back().value > val)
			{
				d.pop_back();
			}
			e.value = val;
			e.d_time = i + k;
			d.push_back(e);
		}
	}
}

void push_val(deque<elmt> &d, int k, int i)
{
	int val;
	elmt e;
	cin >> val;
	if (d.empty())
	{
		e.value = val;
		e.d_time = i + k;
		d.push_back(e);
	}
	else
	{
		while (!d.empty() && d.back().value > val)
		{
			d.pop_back();
		}
		e.value = val;
		e.d_time = i + k;
		d.push_back(e);
	}
}

int makeStep(deque<elmt> &d, int k, int offset)
{
	int val;
	elmt e;
	cin >> val;
	if (d.empty())
	{
		e.value = val;
		e.d_time = offset + k;
		d.push_back(e);
	}
	else
	{
		while (!d.empty() && d.back().value > val)
		{
			d.pop_back();
		}
		e.value = val;
		e.d_time = offset + k;
		d.push_back(e);
	}
	//push_val(d, k, offset);
	while (!d.empty() && d.front().d_time <= offset)
	{
		d.pop_front();
	}

	return d.front().value;
}