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