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#include <bits/stdc++.h>

using namespace std;

#define fi first
#define se second

typedef long long ll;

const int MAXN = 1e6 + 7;

ll a[MAXN];
ll orig[MAXN];

void solve(){

	int n;
	ll k;
	cin >> n >> k;

	for(int i = 1; i <= n; i++){

		cin >> a[i];
		orig[i] = a[i];
	}

	for(int i = 2; i <= n; i++){

		a[i] = max(a[i], a[i - 1] - k);
	}

	for(int i = n - 1; i >= 1; i--){

		a[i] = max(a[i], a[i + 1] - k);
	}

	ll ans = 0;

	for(int i = 1; i <= n; i++){

		ans += (a[i] - orig[i]);
	}

	cout << ans << '\n';
}

bool multi = 0;

int main(){

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

	int t = 1;
	if(multi) cin >> t;
	while(t--){

		solve();
	}

	return 0;
}