n=int(input())
ls1=[]
lsans=[]
flg=0
for i in range(n):
    ls1=list(map(int,input().split()))
    lsans.append(ls1[1]-ls1[0])

if lsans[0] == int(lsans[0]):#整数判定
    if lsans[0]>0:#正判定
        for i in range(2):#同値判定
            if lsans[i-1]!=lsans[i]:
                flg=1
    else:
        flg=1
else:
    flg=1
if flg==1:
    print(-1)
else:
    print(lsans[0])