#include<iostream>
#include<cstdio>
#include<algorithm>
#include<vector>
#include<cstring>
#include<cassert>
#define rep(i, a, b) for(int i = (a); i <= (b); i++)
#define per(i, a, b) for(int i = (a); i >= (b); i--)
using namespace std;
typedef long long ll;
const int N = 100010, INF = 0x3f3f3f3f;
void upd(int &x, int y) {if(x > y) x = y;}
int n, m, ans;
bool flg;
int dfn[N], cd, low[N], stk[N], top, cs, cnt[N];
vector<int> e[N], g[N<<1];

void tarjan(int x){
    dfn[x] = ++cd, low[x] = cd; stk[++top] = x;
    for(int &y : e[x])
        if(!dfn[y]){
            cnt[y]--, cnt[x]++;
            tarjan(y); low[x] = min(low[x], low[y]);
            cnt[x] += cnt[y];
            if(low[y] == dfn[x]){
                g[x].push_back(++cs);
                do g[cs].push_back(stk[top--]);
                while(stk[top+1] != y);
            }
        }
        else if(dfn[y] < dfn[x])
            low[x] = min(low[x], dfn[y]),
            cnt[x]++, cnt[y]--;
    if(cnt[x] > 1) flg = 1;
}

int f[N<<1][8];
bool v[8][8];
void dfs2(int x);
void dfs1(int x){ // circle
    f[x][0] = 0;
    for(int &y : g[x]){
        dfs2(y);
        static int tmp[8];
        memcpy(tmp, f[x], sizeof(tmp));
        memset(f[x], 0x3f, sizeof(f[x]));
        rep(i, 0, 7) if(tmp[i] < INF) rep(j, 0, 7)
            if(v[i][j] && f[y][j] < INF)
                upd(f[x][i|j], tmp[i] + f[y][j]);
    }
}
int h[3][3][2];
void trans(int f[3][2], int g[8]){
    static int tmp[3][2];
    memcpy(tmp, f, sizeof(tmp));
    memset(f, 0x3f, sizeof(tmp));
    rep(i, 0, 2) rep(j, 0, 1) if(tmp[i][j] < INF)
        rep(k, 0, 7) if(g[k] < INF) rep(l, 0, 2)
            if(v[k][1<<i] && v[k|(1<<i)][1<<l])
                upd(f[l][(j+l)&1], tmp[i][j] + g[k] + l);
}
void dfs2(int x){ // square
    if(g[x].size() == 1){
        dfs1(g[x][0]);
        rep(i, 0, 7) if(f[g[x][0]][i] < INF)
            rep(j, 0, 2) if(v[i][1<<j])
                upd(f[x][1<<j], f[g[x][0]][i] + j);
        return;
    }
    for(int &y : g[x]) dfs1(y);
    memset(h, 0x3f, sizeof(h));
    rep(i, 0, 2) h[i][i][i&1] = i;
    for(int &y : g[x]) rep(i, 0, 2) trans(h[i], f[y]);
    rep(i, 0, 2) rep(j, 0, 2) if(v[1<<i][1<<j])
        upd(f[x][(1<<i)|(1<<j)], h[i][j][1]);
}

void solve(){
    memset(cnt, 0, sizeof(cnt));
    memset(f, 0x3f, sizeof(f));
    rep(i, 1, n) e[i].clear(), dfn[i] = 0;
    rep(i, 1, cs) g[i].clear();
    scanf("%d%d", &n, &m);
    cs = n; cd = 0; flg = 0; ans = 0;
    rep(i, 1, m){
        int x, y; scanf("%d%d", &x, &y);
        e[x].push_back(y), e[y].push_back(x);
    }
    rep(i, 1, n) if(!dfn[i]){
        tarjan(i);
        if(flg) {puts("-1"); return;}
        dfs1(i);
        int mn = INF;
        rep(j, 0, 7) upd(mn, f[i][j]);
        ans += mn;
    }
    printf("%d\n", ans);
}

int main(){
    rep(S, 0, 7) rep(T, 0, 7)
        v[S][T] = !(((S&1) && (T>>1&1)) || ((S>>1&1) && (T&1)) || ((S>>2&1) && (T>>2&1)));
    int T = 1; while(T--) solve();
    return 0;
}