N = int(input())
box = [[0 for i in range(2)] for j in range(N)]

for i in range(0, N):
    box[i] = input().split()
    box[i] = [int(i) for i in box[i]]

result = [0 for i in range(N)]
for j in range(0, N):
    result[j] = box[j][1] - box[j][0]

uq = list(set(result))

if len(uq) == 1 and uq[0] > 0:
    print(uq[0])
else:
    print("-1")