n = int(input())

tmp = []
for i in range(n):
    x, y = map(int, input().split())
    z = y - x
    if z  > 0:
        tmp.append(z)
    else:
        tmp.append(-1)

ans = set(tmp)

if len(ans) == 1:
    print(*ans)
else:
    print(-1)