n = int(input())
W = []
for _ in range(n):
    a, b = map(int, input().split())
    W.append(a + 4 * b)
W.sort()
ans = 0
for i in range(n - 1):
    if (W[n - 1] - W[i]) % 2 == 0:
        ans += (W[n - 1] - W[i]) // 2
    else:
        ans = -1
        break
print(ans)