from itertools import combinations


def main():
    N = int(input())

    edges = []
    for i, j in combinations(range(N), 2):
        if abs(i - j) > 0 and i + j < N:
            edges.append((i, j))
    print(len(edges))
    for edge in edges:
        print(edge[0] + 1, edge[1] + 1)


if __name__ == "__main__":
    main()