#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")

#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/rope>

using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;
 
typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;

typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;

typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;

template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define F0R(i, a) for (int i = 0; i < (a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= (a); i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)
#define trav(a, x) for (auto& a : x)

#define mp make_pair
#define pb push_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound

#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rsz resize

const int MOD = 1000000007; // 998244353
const ll INF = 1e18;
const int MX = 100005;
const ld PI = 4*atan((ld)1);

template<class T> void ckmin(T &a, T b) { a = min(a, b); }
template<class T> void ckmax(T &a, T b) { a = max(a, b); }

namespace input {
    template<class T> void re(complex<T>& x);
    template<class T1, class T2> void re(pair<T1,T2>& p);
    template<class T> void re(vector<T>& a);
    template<class T, size_t SZ> void re(array<T,SZ>& a);

    template<class T> void re(T& x) { cin >> x; }
    void re(double& x) { string t; re(t); x = stod(t); }
    void re(ld& x) { string t; re(t); x = stold(t); }
    template<class Arg, class... Args> void re(Arg& first, Args&... rest) { 
        re(first); re(rest...); 
    }

    template<class T> void re(complex<T>& x) { T a,b; re(a,b); x = cd(a,b); }
    template<class T1, class T2> void re(pair<T1,T2>& p) { re(p.f,p.s); }
    template<class T> void re(vector<T>& a) { F0R(i,sz(a)) re(a[i]); }
    template<class T, size_t SZ> void re(array<T,SZ>& a) { F0R(i,SZ) re(a[i]); }
}

using namespace input;

namespace output {
    template<class T1, class T2> void pr(const pair<T1,T2>& x);
    template<class T, size_t SZ> void pr(const array<T,SZ>& x);
    template<class T> void pr(const vector<T>& x);
    template<class T> void pr(const set<T>& x);
    template<class T1, class T2> void pr(const map<T1,T2>& x);

    template<class T> void pr(const T& x) { cout << x; }
    template<class Arg, class... Args> void pr(const Arg& first, const Args&... rest) { 
        pr(first); pr(rest...); 
    }

    template<class T1, class T2> void pr(const pair<T1,T2>& x) { 
        pr("{",x.f,", ",x.s,"}"); 
    }
    template<class T> void prContain(const T& x) {
        pr("{");
        bool fst = 1; for (const auto& a: x) pr(!fst?", ":"",a), fst = 0; // const needed for vector<bool>
        pr("}");
    }
    template<class T, size_t SZ> void pr(const array<T,SZ>& x) { prContain(x); }
    template<class T> void pr(const vector<T>& x) { prContain(x); }
    template<class T> void pr(const set<T>& x) { prContain(x); }
    template<class T1, class T2> void pr(const map<T1,T2>& x) { prContain(x); }
    
    void ps() { pr("\n"); }
    template<class Arg> void ps(const Arg& first) { 
        pr(first); ps(); // no space at end of line
    }
    template<class Arg, class... Args> void ps(const Arg& first, const Args&... rest) { 
        pr(first," "); ps(rest...); // print w/ spaces
    }
}

using namespace output;

namespace io {
    void setIn(string s) { freopen(s.c_str(),"r",stdin); }
    void setOut(string s) { freopen(s.c_str(),"w",stdout); }
    void setIO(string s = "") {
        ios_base::sync_with_stdio(0); cin.tie(0); // fast I/O
        if (sz(s)) { setIn(s+".in"), setOut(s+".out"); } // for USACO
    }
}

using namespace io;

template<class T> T invGeneral(T a, T b) {
    a %= b; if (a == 0) return b == 1 ? 0 : -1;
    T x = invGeneral(b,a); 
    return x == -1 ? -1 : ((1-(ll)b*x)/a+b)%b;
}

template<class T> struct modular {
    T val; 
    explicit operator T() const { return val; }
    modular() { val = 0; }
    template<class U> modular(const U& v) {
        val = (-MOD <= v && v <= MOD) ? v : v % MOD;
        if (val < 0) val += MOD;
    }
    friend ostream& operator<<(ostream& os, const modular& a) { return os << a.val; }
    friend bool operator==(const modular& a, const modular& b) { return a.val == b.val; }
    friend bool operator!=(const modular& a, const modular& b) { return !(a == b); }

    modular operator-() const { return modular(-val); }
    modular& operator+=(const modular& m) { if ((val += m.val) >= MOD) val -= MOD; return *this; }
    modular& operator-=(const modular& m) { if ((val -= m.val) < 0) val += MOD; return *this; }
    modular& operator*=(const modular& m) { val = (ll)val*m.val%MOD; return *this; }
    friend modular exp(modular a, ll p) {
        modular ans = 1; for (; p; p /= 2, a *= a) if (p&1) ans *= a;
        return ans;
    }
    friend modular inv(const modular& a) { 
        auto i = invGeneral(a.val,MOD); assert(i != -1);
        return i;
    } // equivalent to return exp(b,MOD-2) if MOD is prime
    modular& operator/=(const modular& m) { return (*this) *= inv(m); }
    
    friend modular operator+(modular a, const modular& b) { return a += b; }
    friend modular operator-(modular a, const modular& b) { return a -= b; }
    friend modular operator*(modular a, const modular& b) { return a *= b; }
    
    friend modular operator/(modular a, const modular& b) { return a /= b; }
};

typedef modular<int> mi;
typedef pair<mi,mi> pmi;
typedef vector<mi> vmi;
typedef vector<pmi> vpmi;

int N,M,col[MX];
vi adj[MX];
pi cycEdge;
ll ans;

void dfs(int a, int b) {
    trav(t,adj[a]) if (t != b) {
        if (col[t]) cycEdge = {a,t};
        else {
            col[t] = col[a]^3;
            dfs(t,a);
        }
    }
}

int sum[MX];
pi sumCol[MX];

bool bad(int a, int b) {
    return mp(a,b) == cycEdge || mp(b,a) == cycEdge;
}

vpi oops;

void dfs2(int a, int b) {
    sum[a] = 1, sumCol[a].s = (col[a] == 1);
    trav(t,adj[a]) if (t != b && !bad(a,t)) {
        dfs2(t,a);
        sum[a] += sum[t]; 
        sumCol[a].f += sumCol[t].f;
        sumCol[a].s += sumCol[t].s;
        oops.pb({-sumCol[t].f,sum[t]-2*sumCol[t].s});
    }
}

ll eval(int x) {
    ll ans = abs(x);
    trav(t,oops) ans += abs(t.f*x+t.s);
    return ans;
}

int bal[MX];

void dfsBal(int a, int b) {
    trav(t,adj[a]) if (t != b && !bad(a,t)) {
        dfsBal(t,a);
        bal[a] += bal[t];
        ans += abs(bal[t]);
    }
}

int main() {
    setIO(); re(N,M);
    F0R(i,M) {
        int a,b; re(a,b);
        adj[a].pb(b), adj[b].pb(a);
    }
    col[1] = 1;
    dfs(1,0);
    if (cycEdge == mp(0,0) || col[cycEdge.f] != col[cycEdge.s]) {
        if (col[cycEdge.f] != col[cycEdge.s]) {
            sumCol[cycEdge.f].f ++;
            sumCol[cycEdge.s].f --;
        }
        dfs2(1,0);
        if (sum[1] != 2*sumCol[1].s) {
            ps(-1);
            exit(0);
        }
        vi z; z.pb(0);
        trav(t,oops) if (t.f == 1) {
            z.pb(-t.s);
            z.pb(-t.s);
        } else if (t.f == -1) {
            z.pb(t.s);
            z.pb(t.s);
        }
        sort(all(z));
        int res = z[sz(z)/2];
        // ps("WOO",sz(z));
        ps(eval(res));
        exit(0);
    } 
    if (N%2 != 0) {
        ps(-1);
        exit(0);
    }
    // ps(cycEdge,col[cycEdge.f],col[cycEdge.s]);
    int dif = 0;
    FOR(i,1,N+1) {
        if (col[i] == 1) {
            bal[i] = 1; dif ++;
        } else {
            bal[i] = -1; dif --;
        }
    }
    while (dif > 0) {
        ans ++;
        bal[cycEdge.f] --; bal[cycEdge.s] --;
        dif -= 2;
    }
    while (dif < 0) {
        ans ++;
        bal[cycEdge.f] ++; bal[cycEdge.s] ++;
        dif += 2;
    }
    dfsBal(1,0); assert(bal[1] == 0);
    // ps("HA");
    ps(ans);
    //ps(cycEdge,col[cycEdge.f],col[cycEdge.s]);
    //ps(ans);
}


/* stuff you should look for
    * int overflow, array bounds
    * special cases (n=1?), set tle
    * do smth instead of nothing and stay organized
*/

Measured memory_bytes: 15860