def main():
    import sys
    N = int(sys.stdin.readline())
    
    harmonic = 0.0
    for i in range(1, N + 1):
        harmonic += 1.0 / i
    
    term1 = 1.0
    term2 = 2.0 * (harmonic - 1)
    term3 = (N - 2) * (N - 1) * (N + 1) / 12.0
    
    result = term1 + term2 + term3
    print("{0:.10f}".format(result))

if __name__ == "__main__":
    main()