import sys

def main():
    N, M = map(int, sys.stdin.readline().split())
    in_degree = [0] * (N + 1)
    out_degree = [0] * (N + 1)
    for _ in range(M):
        u, v = map(int, sys.stdin.readline().split())
        out_degree[u] += 1
        in_degree[v] += 1

    count_plus = 0
    count_minus = 0
    sum_plus = 0
    sum_minus = 0
    for i in range(1, N+1):
        delta = out_degree[i] - in_degree[i]
        if delta > 0:
            count_plus += 1
            sum_plus += delta
        elif delta < 0:
            count_minus += 1
            sum_minus += -delta

    if sum_plus != sum_minus:
        print(-1)
        return

    if sum_plus == 0:
        print(1)
        return
    else:
        minimal_edges = max(count_plus - 1, count_minus - 1)
        print(minimal_edges)

if __name__ == "__main__":
    main()