#ifndef hari64
#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#define debug(...)
#else
#include "util/viewer.hpp"
#define debug(...) viewer::_debug(__LINE__, #__VA_ARGS__, __VA_ARGS__)
#endif
// clang-format off
using namespace std;
using ll = long long; using ld = long double;
using pii = pair<int, int>; using pll = pair<ll, ll>;
template <typename T> using vc = vector<T>;
template <typename T> using vvc = vector<vc<T>>;
template <typename T> using vvvc = vector<vvc<T>>;
using vi = vc<int>; using vll = vc<ll>; using vpii = vc<pii>; using vpll = vc<pll>;
using vvi = vc<vi>; using vvll = vc<vll>; using vvpi = vc<vpii>; using vvpll = vc<vpll>;
constexpr int INF = 1001001001; constexpr long long INFll = 1001001001001001001;
template <class T> bool chmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }
template <class T> bool chmin(T& a, const T& b) { return a > b ? a = b, 1 : 0; }
// clang-format on

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int N, M;
    cin >> N >> M;

    vector<pair<int, int>> UVs(M);
    vi deg(2 * N, 0);
    for (int i = 0; i < M; i++) {
        int U;
        int V;
        cin >> U >> V;
        U--;
        V--;
        UVs[i] = {U, V};
        deg[U]++;
        deg[N + V]++;
    }

    int oddCnt = 0;
    for (int i = 0; i < N; i++) {
        oddCnt += abs(deg[i] - deg[N + i]);
    }

    debug(oddCnt);

    if (oddCnt == 0) {
        cout << 0 << endl;
    } else {
        if (oddCnt % 2 == 1) {
            cout << -1 << endl;
        } else {
            cout << oddCnt / 2 - 1 << endl;
        }
    }

    return 0;
}