import sys
input=lambda: sys.stdin.readline().rstrip()
n=int(input())
x,y=map(int,input().split())
ans=y-x
for _ in range(n-1):
  x,y=map(int,input().split())
  if x+ans==y:
    continue
  else:
    print(-1)
    break
else:
  print(-1 if ans<=0 else ans)