var
n,i,j:extended;
begin
readln(n);{n<=10^600}
j:=sqrt(1/4+2*n);
i:=-1/2+j;
writeln(trunc(i));

end.

test N6. Error: Crash (floating-point invalid operation)

why??

Thanks for helping

You shouldn't use extended. Use long arithmetic (or another language).