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

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

int n, k, res = 0;
cin >> n >> k;

vector<int> val(n + 1);

for(int i = 0; i < n; i++)
{
    cin >> val[i];
}

for(int i = 1; i < n; i++)
{
    if(val[i - 1] - val[i] > k)
    {
        res += (val[i - 1] - k - val[i]);
        val[i] = val[i - 1] - k;
    }
}

for(int i = n - 2; i >= 0; i--)
{
    if(val[i + 1] - val[i] > k)
    {
        res += (val[i + 1] - k - val[i]);
        val[i] = val[i + 1] - k;
    }
}

cout << res;

return 0;
}
/*
4 2
7 3 0 2
*/