N=int(input())
X = [[int(i) for i in input(). split ()] for i in range(N)]
d=X[0][1]-X[0][0]
if d<=0:
    print (-1)
else:
    nd=1
    i=1
    while nd==1 and i<N:
        d1=X[i][1]-X[i][0]
        if d1!=d or d1<0:
            print (-1)
            nd=0
        i+=1
    if nd==1:
        print(d)