print(-1)
exit()

n, m = map(int,input().split())
din = [0] * n
dout = [0] * n
for i in range(m):
	u, v = map(int,input().split())
	u -= 1
	v -= 1
	din[v] += 1
	dout[u] += 1

tmp = []
for i in range(n):
	tmp.append(dout[i] - din[i])


tmp.sort()
ans = 998244353

# euler graph
ans = 0
for i in range(n):
	if tmp[i] > 0:
		ans += tmp[i]
#ans = min(ans, tar)

if ans == 0:
	print(0)
	exit()

# jun euler graph
ans -= 1

print(ans)