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#include <bits/stdc++.h>
using namespace std;
#define st first
#define nd second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define BOOST ios_base::sync_with_stdio(0), cin.tie(0)
template<typename T>
using vc = vector<T>;
using ll = long long;
using ld = long double;
using ii = pair<int, int>;

int main(){
	BOOST;
	int n, k;
	cin >> n >> k;
	vc<ll> t(n+2);
	for(int i=1; i<=n; i++) cin >> t[i];
	
	ll ans = 0;
	for(int i=1; i<=n; i++){
		if(t[i-1] - t[i] > k){
			ll zwir = t[i-1] - t[i] - k;
			ans += zwir;
			t[i] += zwir;
		}
	}
	for(int i=n; i>=1; i--){
		if(t[i+1] - t[i] > k){
			ll zwir = t[i+1] - t[i] - k;
			ans += zwir;
			t[i] += zwir;
		}
	}

	cout << ans << "\n";
}