N = int(input())
a = [input().split() for i in range(N)]
tmp = int(a[0][1]) - int(a[0][0])
for i in range(N):
    if int(a[i][1]) - int(a[i][0]) != tmp:
        print(-1)
        exit()
    tmp = int(a[i][1]) - int(a[i][0])
if tmp < 1:
    print("-1")
    exit()

print(tmp)