from sys import stdin

def main():
    N = int(input())
    L = stdin.read().splitlines()

    x = [sum(a * 4 if i % 2 else a for i, a in enumerate(map(int, j.split()))) for j in L]
    x.sort(reverse=1)
    ans, maxn = 0, x[0]
    for i in x:
        n = maxn - i
        if n % 2:
            print(-1)
            return
        else:
            ans += n // 2
    print(ans)

main()