program yueyu13;
var
a,w:array[1..10000]of longint;
i,j,l,p,max,n,min:longint;
f:array[0..10000] of longint;
procedure sort(l,r: longint);
var
i,j,x,y: longint;
begin
i:=l;
j:=r;
x:=a[(l+r) div 2];
repeat
while a[i]<x do
inc(i);
while x<a[j] do
dec(j);
if not(i>j) then
begin
y:=a[i];
a[i]:=a[j];
a[j]:=y;
y:=w[i];
w[i]:=w[j];
w[j]:=y;
inc(i);
j:=j-1;
end;
until i>j;
if l<j then
sort(l,j);
if i<r then
sort(i,r);
end;
begin
assign (input,'prisonbreak.in');
reset (input);
assign (output,'prisonbreak.out');
rewrite (output);
readln (n);
for i:=1 to n do
readln (a[i],w[i]);
readln (l,p);
for i:=1 to n do
a[i]:=l-a[i];
sort(1,n);
for j:=0 to n do
f[j]:=-maxlongint;
f[0]:=p;
if p>=a[1] then
f[1]:=f[0]+w[1];
if p>=l then
writeln (0)
else
begin
min:=maxlongint;
for i:=2 to n do
for j:=i downto 1 do
begin
max:=-maxlongint;
if f[j-1]>=a[i] then
max:=f[j-1]+w[i];
if f[j]>=a[i] then
if f[j]>max then
max:=f[j];
f[j]:=max;
if max>=l then
if j<min then
min:=j
end;
if min=maxlongint then
writeln ('-1')
else
writeln (min)
end;
close (input);
close (output)
end.
reamb