for _ in range(int(input())):
    n = int(input())
    a = tuple(map(int, input().split()))
    c = tuple(map(int, input().split()))
    ans = []
    l = False
    for i in range(n - 1):
        n_ans = [a[i]]
        s = a[i] + a[i + 1]
        while s < c[i + 1]:
            if ans:
                s += ans[-1][-1]
                n_ans.append(ans[-1][-1])
                del ans[-1][-1]
                if not ans[-1]:
                    del ans[-1]
            else:
                l = True
                break
        else:
            ans.append(n_ans)
        if l:
            print('-1')
            break
    else:
        print(len(ans))