# -*- coding: utf-8 -*-

N = int(input())
ans = 0
for i in range(N):
    x,y = map(int, input().split())
    if y-x>0:
        if ans == 0:
            ans = y-x
        elif ans != (y-x):
            ans = -1
            break
    else:
        ans = -1
        break

print(ans)