1 #include <cstdio>
 2 #include <iostream>
 3 #include <cstring>
 4 #include <cmath>
 5 using namespace std;
 6 struct edge { int to,from; }e[100005*2];
 7 int n,m,cnt,tot,bz[1005],ans[1005],last[1005],state[1005][2],visit[1005],head,tail,mx;
 8 bool boo;
 9 void insert(int x,int y) { e[++cnt].to=y; e[cnt].from=last[x]; last[x]=cnt; }
10 int abs(int x) { return x<0?-x:x; }
11 void dfs(int x,int k)
12 {
13     if (boo==1) return;
14     bz[x]=k;
15     for (int i=last[x];i;i=e[i].from)
16     {
17         if (boo==1) return;
18         if (bz[e[i].to]==k)
19         {
20             boo=1;
21             return;
22         }
23         if (bz[e[i].to]==-k) continue;
24         dfs(e[i].to,-k);
25     }
26 }
27 int bfs(int x)
28 {
29     memset(visit,0,sizeof(visit));
30     memset(state,0,sizeof(state));
31     head=0; tail=1; state[1][1]=x; visit[x]=1;
32     int r=0;
33     while (head<tail)
34     {
35         head++;
36         int u=state[head][1];
37         r=max(r,state[head][2]);
38         for (int i=last[u];i;i=e[i].from)
39             if (visit[e[i].to]==0)
40             {
41                 visit[e[i].to]=1;
42                 tail++;
43                 state[tail][1]=e[i].to; state[tail][2]=state[head][2]+1;
44             }
45     }
46     return r;
47 }
48 int main()
49 {
50     freopen("merge.in","r",stdin);
51     freopen("merge.out","w",stdout);
52     scanf("%d%d",&n,&m);
53     for (int i=1;i<=m;i++)
54     {
55         int u,v;
56         scanf("%d%d",&u,&v);
57         insert(u,v); insert(v,u);
58     }
59     for (int i=1;i<=n;i++)
60     {
61         if (!bz[i]) dfs(i,++tot);
62         if (boo)
63         {
64             printf("-1\n");
65             return 0;
66         }
67     }
68     for (int i=1;i<=n;i++) ans[abs(bz[i])]=max(ans[abs(bz[i])],bfs(i));
69     for (int i=1;i<=tot;i++) mx+=ans[i];
70     printf("%d",mx);
71     return 0;
72 }