import java.util.*;
class E_3Cycles {
public static void main
(String[] args
) { Scanner s
= new Scanner
(System.
in); int n = s.nextInt();
System.
out.
println(n
/ 2 * (n
- n
/ 2)); for (int i = 1; i <= n / 2; ++i) {
for (int j = n / 2 + 1; j <= n; ++j) {
System.
out.
println(i
+ " " + j
); }
}
}
}
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