#include <bits/stdc++.h>
const double EPS = 1e-9;
const int MOD = 1000003;
const int INF = 2000 * 1000 * 1000;
const int MAX = 4000000;
using namespace std;
int popcount(int n) {
  bitset<32> b(n);
  return b.count();
}
long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }
int lcm(int a, int b) {
  int temp = gcd(a, b);
  return temp ? (a / temp * b) : 0;
}
template <typename T>
T modpow(T base, T exp, T modulus) {
  base %= modulus;
  T result = 1;
  while (exp > 0) {
    if (exp & 1) result = (result * base) % modulus;
    base = (base * base) % modulus;
    exp >>= 1;
  }
  return result;
}
inline bool isInteger(const std::string& s) {
  if (s.empty() || ((!isdigit(s[0])) && (s[0] != '-') && (s[0] != '+')))
    return false;
  char* p;
  strtol(s.c_str(), &p, 10);
  return (*p == 0);
}
int main() {
  long double x1, y1, r1, x2, y2, r2;
  cin >> x1 >> y1 >> r1 >> x2 >> y2 >> r2;
  long double dis =
      hypot(x1 - x2, y1 - y2) >= r1 + r2
          ? (hypot(x1 - x2, y1 - y2) - r1 - r2) / 2
      : hypot(x1 - x2, y1 - y2) < abs(r1 - r2)
          ? (max(r1, r2) - hypot(x1 - x2, y1 - y2) - min(r1, r2)) / 2
          : 0;
  cout << setprecision(20) << dis << endl;
  return 0;
}
