# -*- coding: utf-8 -*-


def main():
    from math import pi
    import sys
    input = sys.stdin.readline

    r = int(input())
    print(2 * pi * r)


if __name__ == '__main__':
    main()

# -*- coding: utf-8 -*-


def main():
    from math import pi
    import sys
    input = sys.stdin.readline

    r = int(input())
    print(2 * pi * r)


if __name__ == '__main__':
    main()

# -*- coding: utf-8 -*-