/// :3
#pragma GCC optimize("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("avx,avx2,fma")
using namespace std;

#include "bits/stdc++.h"

#define pb push_back
#define F first
#define S second
#define f(i, a, b)  for(int i = a; i < b; i++)
#define all(a)  a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define sz(x) (int)(x).size()
#define mp(x,y) make_pair(x,y)
#define popCnt(x) (__builtin_popcountll(x))
//#define int ll

using ll = uint32_t;
using ii = pair<int, int>;
using ull = unsigned long long;

const int N = 3e5+5, LG = 18, MOD = (119 << 23) + 1;
const long double PI = acos(-1);
const long double EPS = 1e-7;
const ll inf = UINT_MAX;
const int BLK = 100;
int n, m, c[N];
int cnt[2];
vector<int> adj[N];
bool dfs(int node, int col) {
    if(~c[node])
        return c[node] == col;
    cnt[col] += 1;
    c[node] = col;
    for(auto v : adj[node]) {
        if(!dfs(v, 1-col))
            return false;
    }
    return true;
}

void doWork() {

    cin >> n >> m;
    memset(c,-1,sizeof c);
    f(i,0,m) {
        int u, v;
        cin >> u >> v;
        adj[u].pb(v);
        adj[v].pb(u);
    }

    int ans = 0;

    f(i,1,n+1)if(c[i] == -1) {
        if(!dfs(i,0)) {
            cout << "-1\n";
            return;
        }
        ans += min(cnt[0], cnt[1]);
        cnt[0] = cnt[1] = 0;
    }

    cout << ans << endl;

}
int32_t main() {
#ifdef ONLINE_JUDGE
    ios_base::sync_with_stdio(0);
    cin.tie(0);
#endif // ONLINE_JUDGE

    int t = 1;
//    cin >> t;
    while (t--) {
        doWork();
    }
    return 0;
}