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#include <bits/stdc++.h>
using namespace std;
#define fwd(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n)    fwd(i, 0, n)
#define all(X)       X.begin(), X.end()
#define sz(X)        int(size(X))
#define pb           push_back
#define eb           emplace_back
#define st           first
#define nd           second
using pii = pair<int, int>;
using vi = vector<int>;
using ll = long long;
using ld = long double;
#ifdef LOC
auto SS = signal(6, [](int) {
	*(int *)0 = 0;
});
#	define DTP(x, y)                                      \
		auto operator<<(auto &o, auto a)->decltype(y, o) { \
			o << "(";                                      \
			x;                                             \
			return o << ")";                               \
		}
DTP(o << a.st << ", " << a.nd, a.nd);
DTP(for (auto i : a) o << i << ", ", all(a));
void dump(auto... x) {
	((cerr << x << ", "), ...) << '\n';
}
#	define deb(x...) cerr << setw(4) << __LINE__ << ":[" #x "]: ", dump(x)
#else
#	define deb(...) 0
#endif

int32_t main() {
	cin.tie(0)->sync_with_stdio(0);
	int n, k;
	cin >> n >> k;
	vi a(n);
	int sum = 0;
	cin >> a[0];
	fwd(i, 1, n) {
		cin >> a[i];
		if (a[i] < a[i - 1]) {
			if (a[i] < a[i - 1] - k) {
				sum += a[i - 1] - k - a[i];
				a[i] = a[i - 1] - k;
			}
		} else {
			for (int j = i - 1; j >= 0; j--) {
				if (a[j] < a[j + 1] - k) {
					sum += a[j + 1] - k - a[j];
					a[j] = a[j + 1] - k;
				} else {
					break;
				}
			}
		}
	}
	cout << sum << '\n';
#ifdef LOCF
	cout.flush();
	cerr << "- - - - - - - - -\n";
	(void)!system(
		"grep VmPeak /proc/$PPID/status | sed s/....kB/\' MB\'/1 >&2"); // 4x.kB
																		// ....kB
#endif
	return 0;
}