procedure sang;
var i,j:longint;
begin
fillchar(prime,sizeof (prime),true);
prime[1]:=false;
for i:=2 to trunc(sqrt(maxn)) do
if prime[i]=true then
begin
j:=i*i;
while j<= maxn do
begin
prime[j]:=false;
j:=j+i;
end;
end;
end;