n = int(input())

l = [list(map(int, input().split())) for _ in range(n)]

l2 = []
for i in l:
    l2.append(i[1] - i[0])

if len(set(l2)) == 1:
    if l2[0] <= 0:
        print(-1)
    else:
        print(l2[0])
else:
    print(-1)