#include <bits/stdc++.h>
using namespace std;

using ll=long long;
#define int ll

#define rng(i,a,b) for(int i=int(a);i<int(b);i++)
#define rep(i,b) rng(i,0,b)
#define gnr(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define per(i,b) gnr(i,0,b)
#define pb push_back
#define eb emplace_back
#define a first
#define b second
#define bg begin()
#define ed end()
#define all(x) x.bg,x.ed
#ifdef LOCAL
#define dmp(x) cerr<<__LINE__<<" "<<#x<<" "<<x<<endl
#else
#define dmp(x) void(0)
#endif

template<class t,class u> void chmax(t&a,u b){if(a<b)a=b;}
template<class t,class u> void chmin(t&a,u b){if(b<a)a=b;}

template<class t> using vc=vector<t>;
template<class t> using vvc=vc<vc<t>>;

using pi=pair<int,int>;
using vi=vc<int>;

template<class t,class u>
ostream& operator<<(ostream& os,const pair<t,u>& p){
	return os<<"{"<<p.a<<","<<p.b<<"}";
}

template<class t> ostream& operator<<(ostream& os,const vc<t>& v){
	os<<"{";
	for(auto e:v)os<<e<<",";
	return os<<"}";
}

#define mp make_pair
#define mt make_tuple
#define one(x) memset(x,-1,sizeof(x))
#define zero(x) memset(x,0,sizeof(x))
#ifdef LOCAL
void dmpr(ostream&os){os<<endl;}
template<class T,class... Args>
void dmpr(ostream&os,const T&t,const Args&... args){
	os<<t<<" ";
	dmpr(os,args...);
}
#define dmp2(...) dmpr(cerr,"Line:",__LINE__,##__VA_ARGS__)
#else
#define dmp2(...) void(0)
#endif

using uint=unsigned;
using ull=unsigned long long;

template<int i,class T>
void print_tuple(ostream&,const T&){
}

template<int i,class T,class H,class ...Args>
void print_tuple(ostream&os,const T&t){
	if(i)os<<",";
	os<<get<i>(t);
	print_tuple<i+1,T,Args...>(os,t);
}

template<class ...Args>
ostream& operator<<(ostream&os,const tuple<Args...>&t){
	os<<"{";
	print_tuple<0,tuple<Args...>,Args...>(os,t);
	return os<<"}";
}

void print(ll x,int suc=1){
	cout<<x;
	if(suc==1)
		cout<<"\n";
	if(suc==2)
		cout<<" ";
}

ll read(){
	ll i;
	cin>>i;
	return i;
}

vi readvi(int n,int off=0){
	vi v(n);
	rep(i,n)v[i]=read()+off;
	return v;
}

template<class T>
void print(const vector<T>&v,int suc=1){
	rep(i,v.size())
		print(v[i],i==int(v.size())-1?suc:2);
}

string readString(){
	string s;
	cin>>s;
	return s;
}

template<class T>
T sq(const T& t){
	return t*t;
}

//#define CAPITAL
void yes(bool ex=true){
	#ifdef CAPITAL
	cout<<"YES"<<endl;
	#else
	cout<<"Yes"<<endl;
	#endif
	if(ex)exit(0);
}
void no(bool ex=true){
	#ifdef CAPITAL
	cout<<"NO"<<endl;
	#else
	cout<<"No"<<endl;
	#endif
	if(ex)exit(0);
}

constexpr ll ten(int n){
	return n==0?1:ten(n-1)*10;
}

const ll infLL=LLONG_MAX/3;

#ifdef int
const int inf=infLL;
#else
const int inf=INT_MAX/2-100;
#endif

int topbit(signed t){
	return t==0?-1:31-__builtin_clz(t);
}
int topbit(ll t){
	return t==0?-1:63-__builtin_clzll(t);
}
int botbit(signed a){
	return a==0?32:__builtin_ctz(a);
}
int botbit(ll a){
	return a==0?64:__builtin_ctzll(a);
}
int popcount(signed t){
	return __builtin_popcount(t);
}
int popcount(ll t){
	return __builtin_popcountll(t);
}
bool ispow2(int i){
	return i&&(i&-i)==i;
}
int mask(int i){
	return (int(1)<<i)-1;
}

bool inc(int a,int b,int c){
	return a<=b&&b<=c;
}

template<class t> void mkuni(vc<t>&v){
	sort(all(v));
	v.erase(unique(all(v)),v.ed);
}

ll rand_int(ll l, ll r) { //[l, r]
	#ifdef LOCAL
	static mt19937_64 gen;
	#else
    static random_device rd;
    static mt19937_64 gen(rd());
    #endif
    return uniform_int_distribution<ll>(l, r)(gen);
}

template<class t>
int lwb(const vc<t>&v,const t&a){
	return lower_bound(all(v),a)-v.bg;
}

struct solvetree{
	vvc<int>g;
	solvetree(vvc<int>gg):g(gg){}
	int ans;
	int dfs(int v,int p,int d){
		int res=d%2?1:-1;
		for(auto c:g[v])if(c!=p)
			res+=dfs(c,v,d+1);
		if(p!=-1)
			ans+=abs(res);
		return res;
	}
	pi slv(int r){
		ans=0;
		int w=dfs(r,-1,0);
		return pi(w,ans);
	}
};

//gr cycles
//rt v->root of the tree
//p par
struct Namori{
	const vvc<int>&g;
	int n;
	vvc<int> gr;
	vi rt,p;
	Namori(const vvc<int>&gg):g(gg),n(g.size()),rt(n),p(n,-1){
		vi c(n);
		rep(i,n)c[i]=g[i].size();
		vi ls;
		rep(i,n)if(c[i]==1)
			ls.pb(i);
		int j=0;
		while(j<int(ls.size())){
			int i=ls[j++];
			for(auto to:g[i])if(c[to]>1){
				p[i]=to;
				if(--c[to]==1)
					ls.pb(to);
			}
		}
		rep(i,n)if(c[i]>1){
			assert(c[i]==2);
			
			int x=i;
			vi v;
			while(x!=-1){
				c[x]=1;
				rt[x]=x;
				v.pb(x);
				int nx=-1;
				for(auto y:g[x])
					if(c[y]==2)
						nx=y;
				x=nx;
			}
			gr.pb(v);
		}
		assert(c==vi(n,1));
		per(ii,ls.size()){
			int i=ls[ii];
			rt[i]=rt[p[i]];
		}
	}
};

void imp(){
	cout<<-1<<endl;
	exit(0);
}

signed main(){
	cin.tie(0);
	ios::sync_with_stdio(0);
	cout<<fixed<<setprecision(20);
	
	int n,m;cin>>n>>m;
	vvc<int> g(n);
	rep(_,m){
		int a,b;cin>>a>>b;
		a--;b--;
		g[a].pb(b);
		g[b].pb(a);
	}
	
	if(n==m){
		Namori w(g);
		vvc<pi> ts(n);
		rep(i,n)if(w.p[i]!=-1)
			ts[w.rt[i]].eb(w.p[i],i);
		assert(w.gr.size()==1);
		vi cyc=w.gr[0];
		
		int ans=0;
		vi x;
		for(auto i:cyc){
			vi vs{i};
			for(auto e:ts[i]){
				vs.pb(e.a);
				vs.pb(e.b);
			}
			mkuni(vs);
			int s=vs.size();
			vvc<int> t(s);
			for(auto e:ts[i]){
				t[lwb(vs,e.a)].pb(lwb(vs,e.b));
			}
			auto res=solvetree(t).slv(lwb(vs,i));
			x.pb(res.a);
			ans+=res.b;
		}
		
		rep(i,x.size())if(i%2)
			x[i]*=-1;
		int s=accumulate(all(x),int(0));
		if(cyc.size()%2==0){
			if(s)imp();
			vvc<int> pos(2*n);
			int cur=n;
			rep(i,x.size()){
				if(x[i]>0){
					rep(_,x[i])
						pos[cur++].pb(i);
				}else{
					rep(_,-x[i])
						pos[--cur].pb(i);
				}
			}
			for(auto z:pos)if(z.size()){
				int len=z.size();
				assert(len%2==0);
				int ad=inf;
				rep(_,2){
					int tmp=0;
					for(int i=0;i<len;i+=2)
						tmp+=z[i+1]-z[i];
					chmin(ad,tmp);
					rotate(z.bg,z.bg+1,z.ed);
					z[len-1]+=x.size();
				}
				ans+=ad;
			}
			cout<<ans<<endl;
		}else{
			if(s%2)imp();
			ans+=abs(s)/2;
			x[0]-=s/2;
			x[x.size()-1]-=s/2;
			int cur=0;
			rep(i,x.size()){
				cur+=x[i];
				ans+=abs(cur);
			}
			assert(cur==0);
			cout<<ans<<endl;
		}
	}else{
		auto ans=solvetree(g).slv(0);
		if(ans.a)imp();
		cout<<ans.b<<endl;
	}
}


Measured memory_bytes: 26180