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//#pragma GCC optimize("Ofast", "unroll-loops")
//#pragma GCC target("sse", "sse2", "sse3", "ssse3", "sse4")

#include <bits/stdc++.h>

#define all(a) a.begin(),a.end()
#define len(a) (int)(a.size())
#define mp make_pair
#define pb push_back
#define fi first
#define se second

using namespace std;

typedef pair<int, int> pii;
typedef long long ll;
typedef long double ld;
template<class T>
using vec = vector<T>;

template<typename T>
bool umin(T &a, T b) {
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}
template<typename T>
bool umax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

#ifdef KoRoVa
#define DEBUG for (bool _FLAG = true; _FLAG; _FLAG = false)
#define LOG(...) print(#__VA_ARGS__" ::", __VA_ARGS__) << endl
template <class ...Ts> auto &print(Ts ...ts) { return ((cerr << ts << " "), ...); }
#else
#define DEBUG while (false)
#define LOG(...)
#endif

const int max_n = -1, inf = 1000111222;

void test_case() {
    int n, k;
    cin >> n >> k;
    vector<int> a(n), lft(n), rht(n);
    for (auto& x: a) cin >> x;
    lft[0] = a[0];
    for (int i = 1; i < n; i++) {
        lft[i] = max(a[i], lft[i - 1] - k);
    }
    rht[n - 1] = a[n - 1];
    for (int i = n - 2; i >= 0; i--) {
        rht[i] = max(a[i], rht[i + 1] - k);
    }
    int ans = 0;
    for (int i = 0; i < n; ++i) {
        ans += max(lft[i], rht[i]) - a[i];
    }
    cout << ans << '\n';
}

int main() {
//    freopen("input.txt", "r", stdin);
//    freopen("output.txt", "w", stdout);

    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int t = 1;

//    cin >> t;

    while (t--) test_case();

    return 0;
}

/*
KoRoVa!
*/