Calculate the sum of a+aa+............+aaaa..a, where a is a natural number less than 10

def demo(v,n):
    assert 0<v<10,'v must between 1 and 9'
    assert type(n)==int,'n must be integer'
    result,t=0,0
    for i in range(n):
        t=t*10+v
        result+=t
    return result

print(demo(3,4))