390B - Inna Dima and Song - CodeForces Solution

n=int(input())
a=[int(i) for i in input().split()]
b=[int(i) for i in input().split()]
ans=0
op=0
from math import *
for i,j in zip(a,b):
    if j==1 or 2*i<j:
        ans-=1
    else:
        op+=1
        if i==j:
            ans+=(i//2)*ceil(i/2)
        elif 2*i==j:
            ans+=i**2
        else:
            ans+=(j//2)*ceil(j/2)
print(-1 if op==0 else ans)