import math

class Vector:
    def __init__(self, v):
        self.vector = v
    def length(self):
        v = self.vector
        return math.sqrt(v[0]*v[0] + v[1]*v[1])
    def normalize(self):
        l = self.length()
        if l != 0:
            self.vector[0] /= l
            self.vector[1] /= l
    def dot(self, v):
        return self.vector[0]*v.vector[0] + self.vector[1]*v.vector[1]
    def angle(self, v):
        return math.acos( self.dot(v) / (self.length() * v.length()))
    def write(self):
        print '('+str(self.vector[0]) + ', ' + str(self.vector[1])+')'