#include<bits/stdc++.h>
#define int long long
using namespace std;
int n,m,ans,val[100005],ban[100005];
vector<int>g[100005];
int dfs(int x,int p,int dep){
	int got=0;
	for(auto to:g[x])if(to!=p&&!ban[to]&&to!=-1){
		int v=dfs(to,x,dep^1);ans+=abs(v);got+=v;
	}
	if(dep&1)got-=val[x];
	else got+=val[x];
	return got;
}
void solve_tree(){
	if(dfs(1,-1,0)){
		cout<<-1<<endl;
		return;
	}
	cout<<ans<<endl;
}
vector<int>cyc;
bool vis[100005];
int par[100005],col[100005],cnt[2];
void go(int x,int p,int dep){
	par[x]=p;cnt[dep&1]++;col[x]=dep&1;vis[x]=1;
	for(auto to:g[x]){
		if(to!=p){
			if(vis[to]){
				if(cyc.size())continue;
				int cur=x;
				while(cur!=to){
					cyc.push_back(cur);
					cur=par[cur];
				}
				cyc.push_back(cur);
			}else{
				go(to,x,dep+1);
			}
		}
	}
}
signed main(){
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		int x,y;cin>>x>>y;
		g[x].push_back(y);
		g[y].push_back(x);
	}
	for(int i=1;i<=n;i++)val[i]=1;
	if(m==n-1){
		solve_tree();
		return 0;
	}else{
		go(1,0,0);
		if(cyc.size()%2==0){
			//even circle
			for(auto vv:cyc)ban[vv]=1;
			vector<int>dat;
			for(auto vv:cyc){
				dat.push_back(dfs(vv,0,col[vv]));
			}
			for(int i=1;i<dat.size();i++)dat[i]+=dat[i-1];
			if(dat.back()){
				puts("-1");
				return 0;
			}
			sort(dat.begin(),dat.end());
			for(int i=0;i<dat.size()/2;i++)ans-=dat[i];
			for(int i=dat.size()/2;i<dat.size();i++)ans+=dat[i];
			cout<<ans<<endl;
		}else{
			//odd circle
			int x=cyc[0],y=cyc.back();
			int dif=cnt[col[x]]-cnt[1-col[x]];
			if(dif%2){
				puts("-1");
				return 0;
			}
			val[x]-=dif/2;val[y]-=dif/2;ans+=abs(dif/2);
			for(auto &v:g[x])if(v==y)v=-1;
			for(auto &v:g[y])if(v==x)v=-1;
			if(dfs(1,-1,0)){
				puts("-1");
				return 0;
			}
			cout<<ans<<endl;
		}
	}
	return 0;
}