#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>
#include <string>
#include <bitset>
#include <iterator>
#include <iomanip>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <ctime>
#include <deque>
#include <queue>
#include <stack>
#include <random>
#include <cassert>

using namespace std;

#define int long long
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()

typedef long long ll;
typedef long double ld;


void solve() {
    int n, x;
    cin >> n >> x;
    vector<int> a(n);
    for (int i = 0; i < n; ++i) cin >> a[i];
    vector<int> r(n);
    r[n - 1] = n - 1 + a[n - 1];
    set<pair<int, int>> cur;
    cur.insert({-(n - 1 + a[n - 1]), n - 1 - a[n - 1]});
    for (int i = n - 2; i >= 0; --i) {
        cur.insert({-(i + a[i]), i - a[i]});
        while ((*cur.begin()).second > i) cur.erase(cur.begin());
        r[i] = -(*cur.begin()).first;
    }
    for (auto &e : r) e = min(e, n - 1);
    int ans = 0;
    vector<int> dp(n + 1);
    for (int i = n - 1; i >= 0; --i) {
        dp[i] = dp[r[i] + 1] + n - 1 - r[i];
    }
    for (auto e : dp) ans += e;
    vector<int> pdp(n + 2);
    for (int i = 0; i <= n; ++i) pdp[i + 1] = pdp[i] + dp[i];
    int tans = ans;
    for (int i = n - 1; i >= 0; --i) {
        int tr = min(n - 1, i + x);
        int tl = max((int)0, i - x);
        if (r[tl] >= tr) continue;
        int L = tl, R = tr + 1;
        while (R - L > 1) {
            int M = (L + R) / 2;
            if (r[M] < tr) L = M;
            else R = M;
        }
        int tans2 = tans - (pdp[R] - pdp[tl]) + dp[tr] * (R - tr) + (R - tr) * (n - 1 - tr);
        // cout << i << ' ' << tans2 << '\n';
        ans = min(ans, tans2);
    }
    cout << ans << '\n';
}

signed main() {
    int tc = 1;
#ifdef LOCAL
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
#else
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
#endif
    // cin >> tc;
    for (int t = 1; t <= tc; t++) {
        // cout << "Case #" << t  << ": ";
        solve();
    }
}