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#include <bits/stdc++.h>
using namespace std;
using ind = long long;
using cind = const ind;
#define FOR(i,l,r) for(int i = (l); i <= (r); i++)
#define FORD(i,l,r) for(int i = (l); i >= (r); i--)

int main(){
ind N, K; cin >> N >> K;
ind v[1'001];
ind sum1=0;
ind sum2=0;
FOR(i,1,N) cin >> v[i];
FOR(i,1,N) sum1 += v[i];
FOR(i,2,N) v[i] = max(v[i], v[i-1] - K);
FORD(i,N-1,1) v[i] = max(v[i], v[i+1] - K);
FOR(i,1,N) sum2 += v[i];
cout << sum2-sum1 << endl;
}