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uses clib;
var
n, a: int16;
b: array of boolean;
begin
readln(n);
setlength(b, n);
for n := n - 1 downto 0 do
b[n] := false;
for n := 1 to length(b) do
begin
read(a);
if (a <= 0) or
(a > length(b)) then
continue;
b[a - 1] := true
end;
if all(b) then
writeln('YES')
else
writeln('NO')
end.
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