N = int(input())
x, y = map(int, input().split())
ans1 = y - x
flag = 0
if ans1 <= 0:
    flag = 1

for i in range(N-1):
    x, y = map(int, input().split())
    ans2 = y - x
    if ans2 <= 0 or ans1 != ans2 :
        flag += 1
    
    ans1 = ans2
    ans2 = 0
    
    if flag != 0 :
        break

if flag == 0:
    print(ans1)
else :
    print(-1)