#include <bits/stdc++.h>
int a[2 * 1000010];
int main() {
  int n, l, t, i, j, nn, k, left;
  scanf("%d%d%d", &n, &l, &t);
  nn = 2 * n;
  for (i = 0; i < n; i++) scanf("%d", &a[i]);
  for (i = n; i < nn; i++) a[i] = l + a[i - n];
  k = 2 * t / l;
  left = 2 * t - k * l;
  double ans = 1.0 * (n - 1) * n * k;
  for (i = 0, j = 0; i < n; i++) {
    while (j < nn && a[j] - a[i] <= left) j++;
    ans += j - 1 - i;
  }
  printf("%lf\n", ans / 4);
  return 0;
}
