N = int(input())
r = [a + 4 * b for a, b in [map(int, input().split()) for _ in range(N)]]

max_num = max(r)
ans = 0

for x in r:
    if(max_num - x) % 2 != 0:
        ans = -1
        break
    ans += (max_num - x) // 2

print(ans)