import sys
N = int(input())
x, y = map(int, input().split())
if y - x <= 0:
    print('-1')
    sys.exit()
ans = y - x
for i in range(N-1):
    x, y = map(int, input().split())
    if y - x <= 0 or ans != y - x:
        print('-1')
        sys.exit()
print(ans)