#include <bits/stdc++.h>
using namespace std;
template <class T, class U>
inline void smin(T &a, U b) {
  if (a > b) a = b;
}
template <class T, class U>
inline void smax(T &a, U b) {
  if (a < b) a = b;
}
template <class T>
inline void gn(T &x) {
  char c, sg = 0;
  while (c = getchar(), (c > '9' || c < '0') && c != '-' && c != -1)
    ;
  if (c == -1) return;
  if (c == '-') sg = 1, c = getchar();
  for (x = 0; c >= '0' && c <= '9';
       x = (x << 1) + (x << 3) + c - '0', c = getchar())
    ;
  if (sg) x = -x;
}
template <class T>
void print(T x) {
  if (x < 0) return putchar('-'), print(-x);
  if (x < 10) {
    putchar(x + '0');
    return;
  }
  print(x / 10);
  putchar(x % 10 + '0');
}
template <class U, class V>
inline void gn(U &u, V &v) {
  gn(u);
  gn(v);
}
template <class U, class V, class W>
inline void gn(U &u, V &v, W &w) {
  gn(u);
  gn(v, w);
}
template <class T>
void println(T &x) {
  print(x);
  putchar('\n');
}
template <class T>
void printsp(T &x) {
  print(x);
  putchar(' ');
}
template <class U, class V>
inline void print(U &u, V &v) {
  printsp(u);
  println(v);
}
template <class U, class V, class W>
inline void print(U &u, V &v, W &w) {
  printsp(u);
  print(v, w);
}
int power(int a, int b, int m, int ans = 1) {
  for (; b; a = (long long)a * a % m, b >>= 1)
    if (b & 1) ans = (long long)ans * a % m;
  return ans;
}
long long a[2 * 1101010];
int main() {
  int n;
  gn(n);
  long long l, t;
  gn(l, t);
  t *= 2;
  for (int i = 0; i < n; i++) gn(a[i]);
  long long k = t / l;
  long long p = t % l;
  double ans = (double)k * n * (n - 1) / 4.0;
  sort(a, a + n);
  for (int i = n; i < 2 * n; i++) a[i] = l + a[i - n];
  for (int i = 0; i < n; i++) {
    int id = upper_bound(a + i, a + n + i, a[i] + p) - a - i - 1;
    ans += (double)id * 0.25;
  }
  printf("%.10lf\n", ans);
  return 0;
}
