N = int(input())
now = 0
flag = 0
for i in range(N):
    x,y = map(int,input().split())
    S = y-x
    if S<0:
        print(-1)
        flag += 1
        break
    else:
        if i == 0:
            now = y-x
        else:
            if S != now:
                print(-1)
                flag += 1
                break
if flag == 0:
    print(now)