n = int(input())
s = [list(map(int, input().split())) for _ in range(n)]
f = s[0][1] - s[0][0]
if f <= 0:
    print(-1)
else:
    for x in s[1:]:
        d = x[1] - x[0]
        if d != f:
            print(-1)
            break
    else:
        print(d)