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

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

    int n, k, a;
    vector<int> path;

    cin >> n >> k;

    int prev = 0;
    int total_trucks = 0;

    for(int i=0; i<n; i++) {
        cin >> a;
        if(a < prev-k) {
            total_trucks += prev-k - a;
            a = prev-k;
        }
        prev = a;
        path.push_back(a);
    }

    prev = 0;

    for(int i=n-1; i>=0; i--) {
        a = path[i];
        if(a < prev-k) {
            total_trucks += prev-k - a;
            a = prev-k;
        }
        prev = a;
    }

    cout<<total_trucks<<"\n";


    return 0;

}