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#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <deque>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

#define JOIN_(X, Y) X##Y
#define JOIN(X, Y) JOIN_(X, Y)
#define TMP JOIN(tmp, __LINE__)
#define PB push_back
#define SZ(x) int((x).size())
#define REP(i, n) for (int i = 0, TMP = (n); i < TMP; ++i)
#define FOR(i, a, b) for (int i = (a), TMP = (b); i <= TMP; ++i)
#define FORD(i, a, b) for (int i = (a), TMP = (b); i >= TMP; --i)

#ifdef DEBUG
#define DEB(x) (cerr << x)
#else
#define DEB(x)
#endif

typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pii;
typedef unsigned int uint;

const int INF = 1e9 + 9;

struct Case {
  int n, k;
  vi t;

  void solve() {
    cin >> n >> k;
    t.resize(n);
    REP(i, n) { cin >> t[i]; }
    int result = 0;
    int required = 0;
    REP(i, n) {
        required -= k;
        if (t[i] < required) {
            result += (required - t[i]);
            t[i] = required;
        }
        required = max(required, t[i]);
    }
    required = 0;
    FORD(i, n - 1, 0) {
        required -= k;
        if (t[i] < required) {
            result += (required - t[i]);
            t[i] = required;
        }
        required = max(required, t[i]);
    }
    cout << result << "\n";
  }
};

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int z = 1;
  // cin >> z;
  while (z--) {
    Case().solve();
  }
}