if __name__ == '__main__':
    numberList=[]
    n=input()
    n=int(n)
    while n>0:
        n-=1
        xy=input()
        x,y=xy.split(' ')
        numberList.append(x)
        numberList.append(y)
    ans=int(numberList[1])-int(numberList[0])
    if ans<0:
        ans=-1
    n=2
    while len(numberList)>n:
        difference=int(numberList[n+1])-int(numberList[n])
        if ans!=difference:
            ans=-1
            break
        else:
            n+=2

    print(ans)