What's wrong with my Verlet Velocity integration code?

Question

Not sure why the code fails on the verlet part. Would appreciate some help! The error that comes up is "IndexError: list index out of range" (the line under the else statement)

from numpy import cos, array
import matplotlib.pyplot as plt
import numpy as np
#constants
N=500
m=1
k=1
dt=0.1
omega=(k/m)**0.5

#initial conditions
x = [0]
v = [0]
t = [0]

## Velocity Verlet
n=0
while t[-1]<=N:
    a=-(k/m)*x[n-1]
    a2=-(k/m)*x[n]
    if n==0:
        v_next=v[n-1]+a*dt
        x_next=x[n-1]+v_next*dt
    else:
        x_next= x[n-1] + dt*v[n-1] + 0.5*(dt**2)*a
        v_next = v[n-1] + 0.5*(a+a2)*dt
    x.append(x_next)
    t.append(t[n-1]+dt)
    n=n+1