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#include <iostream>
#include <vector>

using namespace std;

long long max_value(long long a, long long b) {
    return a > b ? a : b;
}

long long min_value(long long a, long long b) {
    return a < b ? a : b;
}

long long abs_value(long long a) {
    return a < 0 ? -a : a;
}

int main() 
{
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(NULL);

    int n, k;
    int value;

    cin >> n >> k;

    vector<int> sizes(n);
    vector<long long> results(n);

    // Read sizes
    for (int i = 0; i < n; i++) {
        cin >> value;
        sizes[i] = value;
        results[i] = value;
    }

    // Process
    for (int i = 0; i < n; i++) {
        results[i] = sizes[i];
        //cout << i << " " << results[i] << " " << sizes[i] << endl;
        // Before i
        for (int j = 0; j < i; j++) {
            results[i] = max_value(results[i], max_value(sizes[j] - (i - j) * k, 0));
        }
        // After i
        for (int j = i + 1; j < n; j++) {
            results[i] = max_value(results[i], max_value(sizes[j] - (j - i) * k, 0));
        }
        //cout << i << " " << results[i] << " " << sizes[i] << endl;
    }

    long long sum = 0;
    for (int i = 0; i < n; i++) {
        sum += (results[i] - sizes[i]);
    }

    cout << sum << endl;

    return 0;
}