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#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    ll n, k, w=0, i;
    cin >> n >> k;
    
    vector <ll> s1(n), s2(n);
    for (i=0; i<n; ++i){
        cin >> s1[i];
        
        s2[i] = s1[i];
    }
    
    for (i=1; i<n; ++i)
        s2[i] = max(s2[i], s2[i-1] -k);
    for (i=n-2; i>=0; --i)
        s2[i] = max(s2[i], s2[i+1] -k);
    
    for (i=0; i<n; ++i)
        w += s2[i] - s1[i];
    
    cout << w << '\n';

    return 0;
}