N = int(input())
tmp = 0
for i in range(N):
    XY = list(map(int, input().split()))
    if (XY[0] > XY[1]):
        print(-1)
        break
    elif (i == N - 1 and XY[1] - XY[0] == tmp):
        print(tmp)
    else:
        tmp = XY[1] - XY[0]