N = int(input())
for i in range(N):
    x, y = int(input().split())
    difference = y - x
    if i == 0:
        saved_difference = difference
    elif difference <= 0:
        difference = -1
        break
    else:
        if saved_difference != difference:
            difference = -1
            break
        else:
            pass
            
print(difference)