N = int(input())
X = [list(map(int, input().split(' '))) for _ in range(N)]

A = X[0][1] - X[0][0]

for i in (X):
    if i[1] <= i[0]:
        print(-1)
        break
    elif A != i[1] - i[0]:
        print(-1)
        break
else:
     print(i[1] - i[0])