N = int(input())
pairs = []

for n in range(N):
    pair = list(map(int, input().split()))
    pairs.append(pair)

answer = pairs[0][1] - pairs[0][0]

for n in range(1, N):
    z = pairs[n][1] - pairs[n][0]
    if (z != answer):
        answer = -1
        break

if (answer <= 0):
    answer = -1

print(answer)