N = int(input())
a, b = map(int, input().split())
c = b-a
flg = 0
for i in range(N -1):
    a, b = map(int, input().split())
    if c != b-a:
        flg = 1
        
        break

if flg == 1 or c < 0:
    print("-1")
else:
    print(c)