#!/usr/bin/env python3
#fileencoding: utf-8

cand = -1

for i in range(int(input())):
    x, y = [int(i) for i in input().strip().split(" ")]
    if cand == -1:
        cand = y - x
        if cand <= 0:
            cand = -1
            break
        continue
    elif cand != (y-x):
        cand = -1
        break

print(cand)