program EmberAsh;
var
a:array[1..50]of longint;
b:array[1..50]of real;
jour,v,oil,cost,ss,still,t1,t2,t3,buy:real;
i,j,k,n:longint;
fin,fout:text;

BEGIN
assign(fin,'tour.in');reset(fin);
assign(fout,'tour.out');rewrite(fout);
readln(fin,jour);
readln(fin,v,oil,cost,n);
for i:=1 to n do
  readln(fin,a[i],b[i]);
ss:=cost;
still:=v;
t1:=a[1];
t2:=t1/oil;
still:=still-t2;
for i:=2 to n do
  begin
  t1:=a[i]-a[i-1];
  t2:=t1/oil;
  if still<t2 then
    begin
    if b[i]>b[i-1] then
      begin
      buy:=v-still;
      ss:=ss+20+buy*b[i-1];
      still:=v;
      end;
    if b[i]<b[i-1] then
      begin
      t3:=v-still;
      if 20+t3*b[i-1]<20+t3*b[i] then
        begin
        ss:=ss+20+t3*b[i-1];
        still:=v;
        end
          else
          begin

          end;
      end;
    end;
  if still>t2 then
    begin

    end;
  end;
if still<jour-a[n] then
  begin
  t1:=jour-a[n];
  t2:=t1/oil;
  ss:=ss+20+t2*b[n];
  end;
writeln(fout,ss:0:1);
close(fin);
close(fout);
END.