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:
        if XY[1] - XY[0] == tmp:
            print(tmp)
        else:
            print(-1)
    else:
        tmp = XY[1] - XY[0]