Program Task_5;
Var
a : array [1..10000] of integer;
i,n, sum: integer;
check : boolean;
Begin
sum:=0;
read(n);
check:=false;
for i := 1 to n do
       read(a[i]);

for i := 2 to n do
    begin
    if ((a[i-1] > 0) and (a[i] > 0)) then
          check := true;
    if ((a[i-1] < 0) and (a[i] <0)) then
          check := true;
    end;

if (check = true) then
   write('YES')
else
   write('NO');
End.