N = int(input())
ans = 0
ng  = 0
X = [int(i) for i in input().split()]
ans = X[1]-X[0]
for i in range(N-1):
    X = [int(i) for i in input().split()]
    if ans !=  X[1]-X[0]:
        ng = -1
if ans <= 0:
    print("-1")
elif ng == -1:
    print("-1")
else:
    print(ans)