import std.algorithm, std.conv, std.range, std.stdio, std.string;

void main()
{
  auto rd = readln.split.to!(int[]), n = rd[0], p = rd[1].to!long;
  auto h = readln.split.to!(long[]);

  auto calc(int i, int j)
  {
    auto r = 0;
    r += min(h[j]-h[i], p);
    if (j-i > 1 && j < n-1) {
      r += min(h[j]-h[i], p);
    }
    return r;
  }

  auto i = 0, j = 0, ans = 0L;
  while (i < n-1 && j < n-1) {
    if (h[j+1] >= h[j]) {
      ++j;
    } else {
      ans += calc(i, j);
      while (j < n-1 && h[j] > h[j+1]) ++j;
      i = j;
    }
  }
  if (i < j) ans += calc(i, j);
  writeln(ans);
}