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#include <bits/stdc++.h>
using namespace std;
int n,k,a;
long long sum;
int main()
{
    cin>>n>>k;
    vector<int>v,w;
    for(int i=0;i<n;i++){
        cin>>a;
        v.push_back(a);
    }
    w=v;
    for(int i=1;i<n;i++){
        w[i] = max(w[i-1]-k, w[i]);
    }
    for(int i=n-2;i>=0;i--){
        w[i] = max(w[i+1]-k, w[i]);
    }
    for(int i=0;i<n;i++){
        sum += w[i]-v[i];
    }
    cout<<sum;
    return 0;
}