#define DEB
#include<bits/stdc++.h>
#define REP(i,m) for(int i=0;i<(m);++i)
#define REPN(i,m,in) for(int i=(in);i<(m);++i)
#define ALL(t) (t).begin(),(t).end()
#define CLR(a) memset((a),0,sizeof(a))
#define pb push_back
#define mp make_pair
#define fr first
#define sc second

using namespace std;


#ifdef DEB
#define dump(x)  cerr << #x << " = " << (x) << endl
#define prl cerr<<"called:"<< __LINE__<<endl
#define dumpR(x) cerr<<"\x1b[31m"<<#x<<" = " <<(x)<<"\x1b[39m"<<endl
#define dumpY(x) cerr<<"\x1b[33m"<<#x<<" = " <<(x)<<"\x1b[39m"<<endl
#define dumpG(x) cerr<<"\x1b[32m"<<#x<<" = " <<(x)<<"\x1b[39m"<<endl
template<class T> void debug(T a,T b){ for(;a!=b;++a) cerr<<*a<<' ';cerr<<endl;}
#else
#define dump(x) ;
#define dumpR(x) ;
#define dumpY(x) ;
#define dumpG(x) ;
#define prl ;
template<class T> void debug(T a,T b){ ;}
#endif

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

typedef long long int lint;
typedef pair<int,int> pi;

namespace std{
  template<class S,class T>
  ostream &operator <<(ostream& out,const pair<S,T>& a){
    out<<'('<<a.fr<<','<<a.sc<<')';
    return out;
  }
}

//const int INF=5e8;
int n,m;
vector<int> g[100005];
void ng(){
  puts("-1");
  exit(0);
}
pi dfs(int v,int p){
  if(g[v].empty()) return mp(1,1);
  for(auto to:g[v]){
    if(to==p) continue;
    pi nxt=dfs(to,v);
    cost+=nxt.fr;
    sign^=nxt.sc;
  }
  if(sign==0) return mp(cost+1,1);
  else return mp(cost,0);
}

void solve1(){
  REP(i,n-1){
    int a,b;scanf("%d%d",&a,&b);--a;--b;
    g[a].pb(b);
    g[b].pb(a);
  }
  pi res=dfs(0,-1);
  if(res.sc==1) ng();
  printf("%d\n",res.fr);
}


int main(){
  cin>>n>>m;
  if(n&1) ng();
  if(m==n-1) solve1();


  return 0;
}



