{ delphi }

(* Problem:       TABLE
 * Contest:       UOI-2004 (Kharkiv)
 * Type:          Solution
 * Author:        Bogdan Yakovenko
 * Date:          April 1, 2004
 * Language:      Pascal
 * Compiler:      Delphi 5.0
 * Algorithm:     
 *)


{$N+}
const MAXN = 200;
type
  crc = record
          x,y:real;
        end;
var a:array[1..MAXN]of crc;
    b,e:array[1..MAXN]of byte;
    s1,s2:array[1..MAXN]of integer;
    h1,h2,i,j:integer;
    _r,n,gx1,gx2:longint;
    L,R,X:real;
    suxx:boolean;
    o:text;
function dist(i,j:integer):byte;
begin
   if sqrt(sqr(a[i].x-a[j].x)+sqr(a[i].y-a[j].y))-2*(_R+X)<=0 then dist:=1 else dist:=0;
end;

begin
  assign(o,'table.dat');reset(o);
  read(o,gx1,gx2);
  read(o,_R);read(o,N);
  for i:=1 to N do
  begin
    read(o,a[i].x,a[i].y);
  end;
  close(o);
  L:=0;R:=gx2-gx1;
  while abs(L-R)>0.00001 do
  begin
    X:=(L+R)/2;
    {}
    suxx:=false;
    for i:=1 to N do begin b[i]:=0;e[i]:=0;end;
    h1:=0;h2:=0;
    for i:=1 to N do if ((gx1+X)-(a[i].x-_R-X))>0 then
    begin
      inc(h1);
      s1[h1]:=i;
      b[i]:=1;
    end else
    begin
      inc(h2);
      s2[h2]:=i;
    end;

    for i:=1 to N do if ((a[i].x+_R+X)-(gx2-X))>0 then
    begin
      e[i]:=1;if b[i] = 1 then suxx:=true;
    end;
{    oldh2:=0;}
    if not suxx then
    begin
      i:=1;
      while (i<=h1) do
      begin
        j:=1;
        while (j<=h2) do
        begin
          if dist(s1[i],s2[j]) = 1 then
          begin
            if e[s2[j]] = 1 then suxx:=true;
            b[s2[j]]:=1;
            inc(h1);s1[h1]:=s2[j];
            s2[j]:=s2[h2];dec(h2);
          end;
          if suxx then break;
          inc(j);
        end;
        if suxx then break;
        inc(i);
      end;
    end;
    if suxx then R:=X else L:=X;
  end;
  assign(o,'table.sol');rewrite(o);
  writeln(o,2*L:0:3);
  close(o);
end.