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#include <bits/stdc++.h>
using namespace std;
#define fwd(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) fwd(i, 0, n)
#define all(X) X.begin(), X.end()
#define sz(X) int(size(X))
#define pb push_back
#define eb emplace_back
#define st first
#define nd second
using pii = pair<int, int>; using vi = vector<int>;
using ll = long long; using ld = long double;
#ifdef LOC
auto SS = signal(6, [](int) { *(int *)0 = 0; });
#define DTP(x, y) auto operator << (auto &o, auto a) -> decltype(y, o) { o << "("; x; return o << ")"; }
DTP(o << a.st << ", " << a.nd, a.nd);
DTP(for (auto i : a) o << i << ", ", all(a));
void dump(auto... x) { (( cerr << x << ", " ), ...) <<
'\n'; }
#define deb(x...) cerr << setw(4) << __LINE__ << ":[" #x "]: ", dump(x)
#else
#define deb(...) 0
#endif
 

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout << fixed << setprecision(10);
    int n; ll k; cin >> n >> k;
    vector<ll> a(n);
    ll res = 0;
    rep(i, n)cin >> a[i];
    fwd(i, 1, n){
        if(a[i] + k < a[i-1]){
            res += a[i-1] - (a[i] + k);
            a[i] = a[i-1] - k;
        }else{
            int j = i-1;
            while(j >= 0 && a[j] + k < a[j+1]){
                res += a[j+1] - (a[j] + k);
                a[j] = a[j+1] - k;
                j--;
            }
        }
    }
    cout << res << "\n";
    return 0;
}