n = int(input())
c = 0
for i in range(n):
    a, b = map(int, input().split())
    if i == 0:
        C=b-a
        if C<0:
            print(-1)
            break
    else:
        c=b-a
        if C!=c:
            print(-1)
            break
else:
    print(C)