N = int(input())
a, b = map(int, input().split())
S = [a + b * 4]
gf = a % 2
for _ in range(N - 1):
    x, y = map(int, input().split())
    S.append(x + y * 4)
    if x % 2 != gf:
        print(-1)
        break
else:
    print((max(S) * N - sum(S)) // 2)