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#include <iostream>

using namespace std;

using ll = long long;

constexpr ll max_size = 1010;

ll val[max_size];
ll pile[max_size];

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	ll n, k; 
	cin >> n >> k;
	for(int i = 1; i <= n; i++) {
		cin >> val[i];
		pile[i] = val[i];
		pile[i] = max(pile[i], pile[i - 1] - k);
	}
	ll res = 0;
	for(int i = n; i >= 1; i--) {
		pile[i] = max(pile[i], pile[i + 1] - k);
		res += pile[i] - val[i];
	}
	cout << res;
}