N = int(input())
terms = [[int(x) for x in input().split()] for y in range(N)]
forecast = None
is_censored = True

for term in terms:
    base,add = term
    if base >= add:
        break
    fc = add - base
    if not forecast:
        forecast = fc
    else:
        if forecast != fc:
            break
else:
    is_censored = False

if is_censored:
    forecast = -1

print(forecast)