program XemChuaGod;
uses crt;
var i,j,n :integer; A:array[1..100] of integer; KT: boolean;
begin
clrscr;
write('Nhap so n: ');readln(n);
For i:=1 to n do
begin
write('A[',i,']= ');
readln(A[i]);
end;
KT:=true;
if A[1]<A[2] then
begin
For i:=1 to n-2 do
if (A[i]<=A[i+1]) and (A[i+1]<=A[i+2]) then
begin
KT:=false;
break;
end;
end
else
For j:=1 to n-2 do
if (A[j]>=A[j+1]) and (A[j+1]>=A[j+2]) then
begin
KT:=false;
break;
end;
if KT=true then write('YES')
else write('NO');
readln;
end.
