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[-1] - W[I]) % 2 == 0:
        ANS += (W[-1] - W[I]) // 2
    else:
        ANS = -1
        break
print(ANS)