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#include "bits/stdc++.h"
using namespace std;

int const N = 1e3+3;
int n, k, sum, a[N], pfx[N];

int main(){
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cin >> n >> k;
    for(int i = 1; i <= n; ++i) cin >> a[i], pfx[i] = a[i];
    for(int i = 2; i <= n; ++i) pfx[i] = max(pfx[i], pfx[i-1] - k);
    for(int i = n-1; i; --i) pfx[i] = max(pfx[i], pfx[i+1]-k);
    for(int i = 1; i <= n; ++i) sum += pfx[i] - a[i];
    cout << sum << '\n';
}