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#include <bits/stdc++.h>

using namespace std;
#define fors(u, n, s) for(int u = (s); u<(n); u++)
#define foru(u, n) fors(u, n, 0)
#define f first
#define s second
#define vec vector
#define pb push_back
#define sz(x) ((int)x.size())
typedef long long ll;

#define debug(x) cout << __LINE__ << " | " << x << endl
// #define debug(x) {}

void solve() {
	int n, k; cin >> n >> k;
	vec<int> v(n);
	foru(i, n) cin >> v[i];

	vec<int> r(n, INT_MIN);

	int r0 = INT_MIN/2;
	foru(i, n){
		r0 = max(r0-k, v[i]);
		r[i] = max(r[i], r0);
	}

	r0 = INT_MIN/2;
	for(int i = n-1; i>=0; i--){
		r0 = max(r0-k, v[i]);
		r[i] = max(r[i], r0);
	}

	ll ans = 0;
	foru(i, n) ans += (ll)(r[i]-v[i]);
	cout << ans << endl;
}

int main()
{
	// cin.tie(0); cout.tie(0);
	// ios_base::sync_with_stdio(0);

	srand(time(NULL));

	solve();

	return 0;
}