// Slower C++ solution:
#define _CRT_SECURE_NO_WARNINGS

#include <iostream>

#include <sstream>

#include <iomanip>

#include <cstdio>

#include <cstdlib>

#include <cmath>

#include <cctype>

#include <cstring>

#include <vector>

#include <list>

#include <queue>

#include <deque>

#include <stack>

#include <map>

#include <set>

#include <algorithm>

#include <iterator>

#include <bitset>

#include <ctime>

#include<unordered_set>

#include<complex>

using namespace std;



#define FOR(i,a,b) for (int i = (a); i < (b); i++)

#define RFOR(i,b,a) for (int i = (b)-1; i >= (a); i--)

#define ITER(it,a) for (__typeof(a.begin()) it = a.begin(); it != a.end(); it++)

#define FILL(a,value) memset(a, value, sizeof(a))



#define SZ(a) (int)a.size()

#define ALL(a) a.begin(), a.end()

#define PB push_back

#define MP make_pair



typedef long long LL;

typedef vector<int> VI;

typedef pair<int, int> PII;



const double PI = acos(-1.0);

const int INF = 1000 * 1000 * 1000 + 7;

const LL LINF = INF * (LL)INF;



const int MAX = 1000 * 100 + 47;



vector<int> g[MAX];

int C[MAX];

int a = -1;

int b = -1;

bool isLoop;

bool odd;

int VAL[MAX];

int n, m;



void dfs(int v, int c, int p)

{

	C[v] = c;

	FOR(i, 0, SZ(g[v]))

	{

		int to = g[v][i];

		if (to == p) continue;

		if (C[to] != -1)

		{

			if (a == -1)

			{

				isLoop = true;

				if (C[to] != c ^ 1) odd = true;

				a = v;

				b = to;

			}



			continue;

		}



		dfs(to, c ^ 1, v);

	}

}



void imp()

{

	cout << -1 << endl;

	exit(0);

}



void rem(int a, int b)

{

	int ind = -1;

	FOR(i, 0, SZ(g[a]))

	{

		if (g[a][i] == b)

		{

			ind = i;

			break;

		}

	}



	g[a].erase(g[a].begin() + ind);

}



LL ans = 0;

LL dfs1(int v, int p)

{

	LL bal = VAL[v];

	FOR(i, 0, SZ(g[v]))

	{

		int to = g[v][i];

		if (to != p)

		{

			bal += dfs1(to, v);

		}

	}

	

	ans += abs(bal);

	return bal;

}



LL f(int c)

{

	ans = 0;

	VAL[a] += c;

	VAL[b] -= c;

	dfs1(0, 0);

	VAL[a] -= c;

	VAL[b] += c;

	return ans + abs(c);

}



int main()

{

	//freopen("in.txt", "r", stdin);

	//freopen("out.txt", "w", stdout);

	ios::sync_with_stdio(false); cin.tie(0);



	cin >> n >> m;

	FOR(i, 0, m)

	{

		int a, b;

		cin >> a >> b;

		a--;

		b--;

		g[a].push_back(b);

		g[b].push_back(a);

	}



	if (n % 2) imp();

	FILL(C, -1);

	dfs(0, 0, 0);

	int cnt = 0;

	FOR(i, 0, n)

	{

		cnt += C[i];

		VAL[i] = C[i] == 0 ? -1 : 1;

	}



	if (!isLoop)

	{

		if (cnt * 2 != n) imp();

		ans = 0;

		dfs1(0, 0);

		cout << ans << endl;

		return 0;

	}



	rem(a, b);

	rem(b, a);



	if (odd)

	{

		int val = (n - 2 * cnt) / 2;

		VAL[a] += val;

		VAL[b] += val;

		ans = 0;

		dfs1(0, 0);

		cout << ans + abs(val) << endl;

		return 0;

	}



	//FOR(i, 0, n)

	//{

	//	cout << VAL[i] << " ";

	//}

	

	//cout << endl;

	if (cnt * 2 != n) imp();

	int l = -n, r = n;

	while (r - l > 2)

	{

		int l1 = l + (r - l) / 3;

		int r1 = l + 2 * (r - l) / 3;

		if (f(l1) > f(r1))

		{

			l = l1;

		}

		else

		{

			r = r1;

		}

	}



	//cout << f(l) << " " << f(l + 1) << " " << f(l + 2) << endl;

	LL ans = min(f(l), min(f(l + 1), f(l + 2)));

	cout << ans << endl;

}

// Faster C++ solution for the same problem (measured speedup: 20.163060846142507x):
#include<cstdio>

#include<cstring>

#include<algorithm>

#include<cmath>

#define MAXN 100000

using namespace std;

typedef long long LL;

struct node

{

	int to;

	node *nxt;

}edges[MAXN*2+5];

node *ncnt,*Adj[MAXN+5];

int N,M,spx,spy;

LL sum[MAXN+5];

LL cirlen,dist[MAXN+5],xs[MAXN+5];

LL stk[MAXN+5],t;

bool vis[MAXN+5];

void Init()

{

	ncnt=&edges[0];

}

void AddEdge(int u,int v)

{

	node *p=++ncnt;

	p->to=v;

	p->nxt=Adj[u];

	Adj[u]=p;

	

	node *q=++ncnt;

	q->to=u;

	q->nxt=Adj[v];

	Adj[v]=q;

}

void DFS(int u,int fa)

{

	vis[u]=true;

	for(node *p=Adj[u];p!=NULL;p=p->nxt)

	{

		int v=p->to;

		if(v==fa)

			continue;

		if(vis[v]==true)

		{

			spx=u,spy=v;

			cirlen=dist[v]-dist[u]+1;

			continue;

		}

		dist[v]=dist[u]+1;

		DFS(v,u);

	}

}

void GetXs(int u,int fa)

{

	vis[u]=true;

	for(node *p=Adj[u];p!=NULL;p=p->nxt)

	{

		int v=p->to;

		if(v==fa||vis[v]==true)

			continue;

		GetXs(v,u);

		xs[u]+=xs[v];

		sum[u]+=sum[v];

	}

}

int main()

{

//	freopen("print.in","r",stdin);

//	freopen("print.out","w",stdout);

	Init();

	scanf("%d %d",&N,&M);

	int u,v;

	for(int i=1;i<=M;i++)

	{

		scanf("%d %d",&u,&v);

		AddEdge(u,v);

	}

	dist[1]=0;

	DFS(1,-1);

	LL ans=0,tot=0;

	for(int i=1;i<=N;i++)

	{

		if(dist[i]%2==0)

			sum[i]=1LL;

		else

			sum[i]=-1LL;

		tot+=sum[i];

	}

	if(N-1==M)

	{

		if(tot!=0)

		{

			printf("-1\n");

			return 0;

		}

	}

	else

	{

		if(cirlen%2==1)

		{

			if(tot%2==1)

			{

				printf("-1\n");

				return 0;

			}

			sum[spx]-=tot/2;

			sum[spy]-=tot/2;

			ans+=abs(tot)/2;

		}

		else

		{

			if(tot!=0)

			{

				printf("-1\n");

				return 0;

			}

			xs[spx]=1LL;

			xs[spy]=-1LL;

		}

	}

	memset(vis,0,sizeof(vis));

	GetXs(1,-1);

	for(int i=1;i<=N;i++)

	{

		if(xs[i]==0)	ans+=abs(sum[i]);

		else

		{

			if(xs[i]==1)

				sum[i]=-sum[i];

			stk[++t]=sum[i];

		}

	}

	stk[++t]=0;

	sort(stk+1,stk+1+t);

	LL k=stk[(t+1)/2];

	for(int i=1;i<=t;i++)

		ans+=1LL*abs(stk[i]-k);

	printf("%lld\n",ans);

	return 0;

}