import sys

def main():
    N = int(sys.stdin.readline())

    edges = []
    # Use 0-indexed vertices for logic (0 to N-1), 
    # then convert to 1-indexed for output (1 to N).
    
    # Connect vertex i and vertex j if i < j and i + j >= N - 1
    for i in range(N):
        for j in range(i + 1, N): # Ensure i < j to avoid self-loops and duplicate edges
            if i + j >= N - 1:
                edges.append((i + 1, j + 1)) # Add 1 for 1-indexed output

    print(len(edges))
    for u, v in edges:
        print(u, v)

if __name__ == '__main__':
    main()