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

int main() {
    int n,k;
    cin>>n>>k;
    vector<int> v;
    for(int i=0;i<n;i++) {
        int x;
        cin>>x;
        v.push_back(x);
    }
    int s = 0;
    for (int i = 1; i<n; i++){
        if (v[i-1] > v[i]) {
            if (v[i-1]-v[i] > k) {
                s += v[i-1]-v[i]-k;
                v[i] += (v[i-1]-v[i]-k);
            }
        }
    }
    for (int i = n-2; i>=0; i--) {
        if (v[i+1] > v[i] ) {
            if (v[i+1]-v[i] > k) {
                s += v[i+1]-v[i]-k;
                v[i] += (v[i+1]-v[i]-k);
            }
        }
    }
    cout << s << endl;
}