r

def r(l):
    def x(i):
        return l[i][0]

    def y(i):
        return l[i][1]

    xs = [x(i) for i in range(len(l))]
    ys = [y(i) for i in range(len(l))]

    r = (
          (len(l) * sum([x(i) * y(i) for i in range(len(l))]))
        - (sum(xs) * sum(ys))
        ) / ((
          (
            len(l) * sum([x(i) ** 2 for i in range(len(l))])
          - (sum(xs) ** 2)
          ) * (
            len(l) * sum([y(i) ** 2 for i in range(len(l))])
          - (sum(ys) ** 2)
          )
        ) ** 0.5)

    return r