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

if XY[0][0]>=XY[0][1]:
    print(-1)
    exit()

now = XY[0][1]-XY[0][0]

for i in range(1,N):
    if XY[i][1]-XY[i][0]!=now:
        print(-1)
        exit()

print(now)