import sys
from typing import Counter
readline=sys.stdin.readline

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