#!/usr/bin/env python
# -*- coding: utf-8 -*-

N = int(input())
X,Y = map(int,input().split())
ans = Y-X
for i in range(N-1):
    X,Y = map(int,input().split())
    if ans != Y-X:
        break
else:
    print(ans if ans > 0 else -1)
    quit()
print(-1)