// clang-format off
#ifdef _LOCAL
    #include <pch.hpp>
#else
    #include <bits/stdc++.h>
    #define cerr if (false) cerr
    #define debug_bar
    #define debug(...)
    #define debug2(vv)
    #define debug3(vvv)
#endif

using namespace std;
using ll = long long;
using ld = long double;
using str = string;
using P = pair<ll,ll>;
using VP = vector<P>;
using VVP = vector<VP>;
using VC = vector<char>;
using VS = vector<string>;
using VVS = vector<VS>;
using VI = vector<int>;
using VVI = vector<VI>;
using VVVI = vector<VVI>;
using VLL = vector<ll>;
using VVLL = vector<VLL>;
using VVVLL = vector<VVLL>;
using VB = vector<bool>;
using VVB = vector<VB>;
using VVVB = vector<VVB>;
using VD = vector<double>;
using VVD = vector<VD>;
using VVVD = vector<VVD>;
#define FOR(i,l,r) for (ll i = (l); i < (r); ++i)
#define RFOR(i,l,r) for (ll i = (r)-1; (l) <= i; --i)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define FORE(e,c) for (auto&& e : c)
#define ALL(c) (c).begin(), (c).end()
#define SORT(c) sort(ALL(c))
#define RSORT(c) sort((c).rbegin(), (c).rend())
#define MIN(c) *min_element(ALL(c))
#define MAX(c) *max_element(ALL(c))
#define COUNT(c,v) count(ALL(c),(v))
#define len(c) ((ll)(c).size())
#define BIT(b,i) (((b)>>(i)) & 1)
#define PCNT(b) ((ll)__builtin_popcountll(b))
#define LB(c,v) distance((c).begin(), lower_bound(ALL(c), (v)))
#define UB(c,v) distance((c).begin(), upper_bound(ALL(c), (v)))
#define UQ(c) do { SORT(c); (c).erase(unique(ALL(c)), (c).end()); (c).shrink_to_fit(); } while (0)
#define END(...) do { print(__VA_ARGS__); exit(0); } while (0)
constexpr ld EPS = 1e-10;
constexpr ld PI  = acosl(-1.0);
constexpr int inf = (1 << 30) - (1 << 15);   // 1,073,709,056
constexpr ll INF = (1LL << 62) - (1LL << 31);  // 4,611,686,016,279,904,256
template<class... T> void input(T&... a) { (cin >> ... >> a); }
void print() { cout << '\n'; }
template<class T> void print(const T& a) { cout << a << '\n'; }
template<class P1, class P2> void print(const pair<P1, P2>& a) { cout << a.first << " " << a.second << '\n'; }
template<class T, class... Ts> void print(const T& a, const Ts&... b) { cout << a; (cout << ... << (cout << ' ', b)); cout << '\n'; }
template<class T> void cout_line(const vector<T>& ans, int l, int r) { for (int i = l; i < r; i++) { if (i != l) { cout << ' '; } cout << ans[i]; } cout << '\n'; }
template<class T> void print(const vector<T>& a) { cout_line(a, 0, a.size()); }
template<class S, class T> bool chmin(S& a, const T b) { if (b < a) { a = b; return 1; } return 0; }
template<class S, class T> bool chmax(S& a, const T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> T SUM(const vector<T>& A) { return accumulate(ALL(A), T(0)); }
template<class T> vector<T> cumsum(const vector<T>& A, bool offset = false) { int N = A.size(); vector<T> S(N+1, 0); for (int i = 0; i < N; i++) { S[i+1] = S[i] + A[i]; } if (not offset) { S.erase(S.begin()); } return S; }
template<class T> string to_binary(T x, int B = 0) { string s; while (x) { s += ('0' + (x & 1)); x >>= 1; } while ((int)s.size() < B) { s += '0'; } reverse(s.begin(), s.end()); return s; }
template<class F> ll binary_search(const F& is_ok, ll ok, ll ng) { while (abs(ok - ng) > 1) { ll m = (ok + ng) / 2; (is_ok(m) ? ok : ng) = m; } return ok; }
template<class F> double binary_search_real(const F& is_ok, double ok, double ng, int iter = 90) { for (int i = 0; i < iter; i++) { double m = (ok + ng) / 2; (is_ok(m) ? ok : ng) = m; } return ok; }
template<class T> using PQ_max = priority_queue<T>;
template<class T> using PQ_min = priority_queue<T, vector<T>, greater<T>>;
template<class T> T pick(stack<T>& s) { assert(not s.empty()); T x = s.top(); s.pop(); return x; }
template<class T> T pick(queue<T>& q) { assert(not q.empty()); T x = q.front(); q.pop(); return x; }
template<class T> T pick_front(deque<T>& dq) { assert(not dq.empty()); T x = dq.front(); dq.pop_front(); return x; }
template<class T> T pick_back(deque<T>& dq) { assert(not dq.empty()); T x = dq.back(); dq.pop_back(); return x; }
template<class T> T pick(PQ_min<T>& pq) { assert(not pq.empty()); T x = pq.top(); pq.pop(); return x; }
template<class T> T pick(PQ_max<T>& pq) { assert(not pq.empty()); T x = pq.top(); pq.pop(); return x; }
template<class T> T pick(vector<T>& v) { assert(not v.empty()); T x = v.back(); v.pop_back(); return x; }
int to_int(const char c) { if (islower(c)) { return (c - 'a'); } if (isupper(c)) { return (c - 'A'); } if (isdigit(c)) { return (c - '0'); } assert(false); }
char to_a(const int i) { assert(0 <= i && i < 26); return ('a' + i); }
char to_A(const int i) { assert(0 <= i && i < 26); return ('A' + i); }
char to_d(const int i) { assert(0 <= i && i <= 9); return ('0' + i); }
ll min(int a, ll b) { return min((ll)a, b); }
ll min(ll a, int b) { return min(a, (ll)b); }
ll max(int a, ll b) { return max((ll)a, b); }
ll max(ll a, int b) { return max(a, (ll)b); }
ll mod(ll x, ll m) { assert(m > 0); return (x % m + m) % m; }
ll ceil(ll a, ll b) { if (b < 0) { return ceil(-a, -b); } assert(b > 0); return (a < 0 ? a / b : (a + b - 1) / b); }
ll floor(ll a, ll b) { if (b < 0) { return floor(-a, -b); } assert(b > 0); return (a > 0 ? a / b : (a - b + 1) / b); }
ll powint(ll x, ll n) { assert(n >= 0); if (n == 0) { return 1; }; ll res = powint(x, n>>1); res *= res; if (n & 1) { res *= x; } return res; }
pair<ll,ll> divmod(ll a, ll b) { assert(b != 0); ll q = floor(a, b); return make_pair(q, a - q * b); }
ll bitlen(ll b) { if (b <= 0) { return 0; } return (64LL - __builtin_clzll(b)); }
ll digitlen(ll n) { assert(n >= 0); if (n == 0) { return 1; } ll sum = 0; while (n > 0) { sum++; n /= 10; } return sum; }
ll msb(ll b) { return (b <= 0 ? -1 : (63 - __builtin_clzll(b))); }
ll lsb(ll b) { return (b <= 0 ? -1 : __builtin_ctzll(b)); }
// --------------------------------------------------------

// References:
//   <https://github.com/atcoder/ac-library/blob/v1.4/atcoder/dsu.hpp>
//   <https://en.wikipedia.org/wiki/Disjoint-set_data_structure>

// Disjoint-set data structure (Union Find)
// 
struct dsu {
  public:
    dsu() : N(0) {}
    explicit dsu(int n) : N(n), parent_or_size(n, -1), n_edge(n, 0) {}

    // 辺 (a, b) を張ってマージ成否を返す : amortized O(α(N))
    bool merge(int a, int b) {
        assert(0 <= a && a < N);
        assert(0 <= b && b < N);
        int x = leader(a), y = leader(b);
        if (x == y) { n_edge[x]++; return false; }
        if (-parent_or_size[x] < -parent_or_size[y]) { swap(x, y); }
        parent_or_size[x] += parent_or_size[y];
        parent_or_size[y] = x;
        n_edge[x] += n_edge[y] + 1;
        return true;
    }

    // 頂点 a, b が連結か判定する : amortized O(α(N))
    bool same(int a, int b) {
        assert(0 <= a && a < N);
        assert(0 <= b && b < N);
        return leader(a) == leader(b);
    }

    // 頂点 a の属する連結成分のルートを返す : amortized O(α(N))
    int leader(int a) {
        assert(0 <= a && a < N);
        if (parent_or_size[a] < 0) { return a; }
        return parent_or_size[a] = leader(parent_or_size[a]);
    }

    // 頂点 a が属する連結成分のサイズを返す : amortized O(α(N))
    int size(int a) {
        assert(0 <= a && a < N);
        return -parent_or_size[leader(a)];
    }

    // a が属する連結成分の辺の数を返す : amortized O(α(N))
    int size_e(int a) {
        assert(0 <= a && a < N);
        return n_edge[leader(a)];
    }

    // 「一つの連結成分の頂点番号リスト」のリストを返す : O(N)
    vector<vector<int>> groups() {
        vector<int> leader_buf(N), group_size(N);
        for (int i = 0; i < N; i++) {
            leader_buf[i] = leader(i);
            group_size[leader_buf[i]]++;
        }
        vector<vector<int>> result(N);
        for (int i = 0; i < N; i++) {
            result[i].reserve(group_size[i]);
        }
        for (int i = 0; i < N; i++) {
            result[leader_buf[i]].push_back(i);
        }
        result.erase(
            remove_if(result.begin(), result.end(),
                      [&](const vector<int>& v) { return v.empty(); }),
            result.end());
        return result;
    }

  private:
    int N;
    // [x < 0] -x が連結成分のサイズに対応
    // [0 <= x] x が parent に対応
    vector<int> parent_or_size;
    vector<int> n_edge;
};

// clang-format on
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);

    int N, M;
    cin >> N >> M;
    VVI G(N);
    VI deg(N);
    dsu uf(N);
    REP (_, M) {
        int u, v;
        cin >> u >> v;
        u--;
        v--;
        G[u].push_back(v);
        deg[u]--;
        deg[v]++;
        uf.merge(u, v);
    }

    if (N == 1) { END(0); }
    if (SUM(deg) & 1) { END(-1); }
    debug(deg);

    ll L = 0, R = 0;
    REP (u, N) {
        if (deg[u] < 0) { L += deg[u]; }
        if (deg[u] > 0) { R += deg[u]; }
    }

    ll C = 0;
    ll D = 0;
    FORE(g, uf.groups()) {
        if (uf.size_e(g[0])) { C++; }
        bool ok = false;
        FORE(u, g) {
            if (deg[u] != 0) { ok = true; }
        }
        D += ok;
    }

    ll ans = 0;
    if (R == 0) {
        ans = C - 1;
    } else {
        // ans = R - 1 + C - 1;
        ans = R - 1 + C - D;
    }
    // ll ans = max({0ll, R - 1, C - 1});
    print(ans);

    return 0;
}