#include <stdio.h>
int n,l,t,x,i,j,a[2000200];
double r;
int main() {
scanf("%d%d%d",&n,&l,&t);
for (i=0; i<n; i++) {
    scanf("%d",&a[i]);
    a[n+i]=a[i]+l;
}
t*=2; x=t/l; t%=l;
for (i=0; i<n; i++) {
    for (; j<2*n && a[j]-a[i]<=t; j++);
    r+=j-i-1;
}
printf("%.8lf\n",0.25*(r+double(x)*double(n)*(n-1.)));
return 0;
}