class Particle:
    def __init__(self, mass=1.0, velocity=None, position=None):
        self.mass = mass
        self.velocity = list(velocity if velocity is not None else [0.0, 0.0, 0.0])
        self.position = list(position if position is not None else [0.0, 0.0, 0.0])

    def __repr__(self):
        return f"Particle(mass={self.mass}, velocity={self.velocity}, position={self.position})"


class cloud:
    def __init__(self, particles=None):
        self.particles = list(particles or [])

    def add(self, p):
        self.particles.append(p)

    def step(self, dt=1.0):
        for p in self.particles:
            p.position = [x + v * dt for x, v in zip(p.position, p.velocity)]
        return self

    def center(self):
        total = sum(p.mass for p in self.particles)
        if total == 0:
            return [0.0, 0.0, 0.0]
        answer = [0.0, 0.0, 0.0]
        for p in self.particles:
            answer = [a + p.mass * x for a, x in zip(answer, p.position)]
        return [x / total for x in answer]

    def __repr__(self):
        return f"cloud({self.particles})"