n = int(input())
v = []
for i in range(n):
    a, b = map(int, input().split())
    v.append(a + 4 * b)

ans = 0
mx = max(v)
for i in range(n):
    if (mx - v[i]) % 2 != 0:
        print(-1)
        exit()
    ans += (mx - v[i]) // 2
print(ans)