n, m = map(int, input().split())
out_degree = [0] * (n + 1)
in_degree = [0] * (n + 1)

for _ in range(m):
    u, v = map(int, input().split())
    out_degree[u] += 1
    in_degree[v] += 1

sum_delta = 0
sum_abs = 0

for i in range(1, n + 1):
    delta = out_degree[i] - in_degree[i]
    sum_delta += delta
    sum_abs += abs(delta)

if sum_delta != 0:
    print(-1)
else:
    if sum_abs == 0:
        print(0)
    elif sum_abs == 2:
        print(0)
    else:
        if sum_abs % 2 != 0:
            print(-1)
        else:
            add = (sum_abs - 2) // 2
            print(add)