var a:array[1..100000] of longint;
  b,c,i:longint;
begin
read(b);
for i:=1 to b do
read(a[i]);
read(c);
for i:=b downto 1 do
if a[i]<>c then b:=b-1
else break;
if b=0 then write('-1')
else write(b);
end.
/**************************************************************
	Problem: 1815
	User: admin
	Language: Pascal
	Result: Wrong Answer
****************************************************************/