#include<bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define maxn 200005
#define put() putchar('\n')
#define Tp template<typename Ty>
#define Ts template<typename Ty,typename... Ar>
using namespace std;
void read(int &x){
    int f=1;x=0;char c=getchar();
    while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
    while (c>='0'&&c<='9') {x=x*10+c-'0';c=getchar();}
    x*=f;
}
namespace Debug{
	Tp void _debug(char* f,Ty t){cerr<<f<<'='<<t<<endl;}
	Ts void _debug(char* f,Ty x,Ar... y){while(*f!=',') cerr<<*f++;cerr<<'='<<x<<",";_debug(f+1,y...);}
	Tp ostream& operator<<(ostream& os,vector<Ty>& V){os<<"[";for(auto& vv:V) os<<vv<<",";os<<"]";return os;}
	#define gdb(...) _debug((char*)#__VA_ARGS__,__VA_ARGS__)
}using namespace Debug;
#define fi first
#define se second
#define mk make_pair
const int mod=1e9+7;
int power(int x,int y=mod-2) {
	int sum=1;
	while (y) {
		if (y&1) sum=sum*x%mod;
		x=x*x%mod;y>>=1;
	}
	return sum;
}
vector<int>to[maxn];
int n,m,dfn[maxn],deep[maxn],fl1,fl2,dp[maxn],ans;
void dfs(int x,int pre) {
	dfn[x]=1;deep[x]=deep[pre]+1;
	int Max=0,Maxx=0;
	for (auto y:to[x]) if (y^pre) {
		if (!dfn[y]) {
			dfs(y,x);
			if (Max<=dp[y]) Maxx=Max,Max=dp[y];
			else if (Maxx<=dp[y]) Maxx=dp[y];
			dp[x]=max(dp[x],dp[y]+1);
		}
		else {
			if ((deep[y]-deep[x])%2) fl1=max(fl1,deep[y]-deep[x]+1);
			else fl2=max(fl2,deep[y]-deep[x]+1);
		}
	}
	ans=max(ans,Max+Maxx+1);
}
signed main(void){
//	freopen("1.in","r",stdin);
	int i,x,y;
	read(n);read(m);
	if (n<=3) return puts("-1"),0;
	for (i=1;i<=m;i++) {
		read(x),read(y);
		to[x].push_back(y);
		to[y].push_back(x);
	}
	for (i=1;i<=n;i++) if (!dfn[i]) dfs(i,0);
	if (fl1&&fl2) puts("0");
	else if (fl1) {
		if (fl1==3&&ans<=3) printf("2");
		else printf("1");
	}
	else if (fl2) {
		printf("1");
	}
	else printf("%d",max(4-ans,0)+2);
	return 0;
}