n=int(input())
l=[list(map(int,input().split())) for i in range(n)]
ans=[]
for i in range(n):
  ans.append(l[i][1]-l[i][0])
ans=list(set(ans))
if len(ans)==1 and ans[0]>0:
  print(ans[0])
else:
  print(-1)