import sys 
input = sys.stdin.buffer.readline
def main():
    n = int(input())
    G = [[] for _ in range(n)]
    req = n - 1
    for v in range(n):
        for u in range(v+1,n):
            if len(G[v]) >= req:
                break
            G[v].append(u)
            G[u].append(v)
        req -= 1
    ans = 0
    for i in range(n):
        ans += len(G[i])
    ans = ans // 2
    print(ans)
    for v in range(n):
        for u in G[v]:
            if u > v:
                ret = [v+1, u+1]
                print(*ret)


if __name__ == '__main__':
    main()