N = int(input())
L = []
counter = 0
for i in range(N) :
    X,Y = map(int,input().split())
    Z = Y-X
    if Z > 0 :
        L.append(Z)
    else :
        counter += 1
        L.append(Z)
a = L[0]
b = L.count(a)
if b == N and counter == 0 :
    print(a)
else :
    print(-1)