#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int maxn = 100005;
int a[3], d;
void Solve() {
  cin >> a[0] >> a[1] >> a[2] >> d;
  sort(a, a + 3);
  long long res = 0;
  if (a[2] < a[1] + d) res += a[1] + d - a[2];
  if (a[0] > a[1] - d) res += a[0] - a[1] + d;
  cout << res;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  if (fopen(""
            ".inp",
            "r")) {
    freopen(
        ""
        ".inp",
        "r", stdin);
    freopen(
        ""
        ".out",
        "w", stdout);
  }
  int test_case = 1;
  while (test_case--) Solve();
  return 0;
}
