## https://yukicoder.me/problems/no/2403


def main():
    N, M = map(int, input().split())
    in_degrees = [0] * N
    out_degrees = [0] * N

    for _ in range(M):
        u, v = map(int, input().split())
        in_degrees[u - 1] += 1
        out_degrees[v - 1] += 1
    
    in_degree_total = 0
    out_degree_total = 0
    for i in range(N):
        if in_degrees[i] > out_degrees[i]:
            in_degree_total += in_degrees[i] - out_degrees[i]
        elif in_degrees[i] < out_degrees[i]:
            out_degree_total += out_degrees[i] - in_degrees[i]
    
    if in_degree_total != out_degree_total:
        print(-1)
        return
    
    a = max(0, in_degree_total - 1)
    print(a)







if __name__ == "__main__":
    main()