def main():
    import sys
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx])
    idx += 1
    M = int(input[idx])
    idx += 1

    out_degree = [0] * (N + 1)
    in_degree = [0] * (N + 1)

    for _ in range(M):
        u = int(input[idx])
        idx += 1
        v = int(input[idx])
        idx += 1
        out_degree[u] += 1
        in_degree[v] += 1

    d = [0] * (N + 1)
    for i in range(1, N + 1):
        d[i] = out_degree[i] - in_degree[i]

    sum_d = sum(d)
    if sum_d != 0:
        print(-1)
        return

    non_zero = []
    for i in range(1, N + 1):
        if d[i] != 0:
            non_zero.append(d[i])

    cnt_nonzero = len(non_zero)
    if cnt_nonzero == 0:
        print(0)
        return
    elif cnt_nonzero == 2:
        a, b = non_zero
        if (a == 1 and b == -1) or (a == -1 and b == 1):
            print(0)
        else:
            print(-1)
        return
    else:
        S = sum(x for x in non_zero if x > 0)
        if S % 2 != 0:
            print(-1)
        else:
            print(S // 2)

if __name__ == "__main__":
    main()