#include <bits/stdc++.h>
#include <unordered_set>
using namespace std;

#define st first
#define nd second
#define pb push_back
#define mp make_pair
#define klar(v) memset(v, 0, sizeof(v))
#define bust ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define gcd(a, b) __gcd(a, b);
#define debug cout << "chuj" << endl;
#define endl "\n"

typedef vector<int> vi;
typedef vector<pair<int, int> > vpii;
typedef vector<long long> vll;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
typedef long long ll;
const int mamm = 3e3+200, maxn = 2e3+10, inf = 10000000;

vi graph[3010];
int odl[3010];
bool done[3010];
bool odw[3010];
priority_queue <int> chuj;
pii bfs(int v){
    queue <int> que;
    que.push(v);
    pii ret = mp(-1, -1);
    odw[v] = 1;
    while(que.size()){
        int v = que.front(); que.pop();
//        cout << v << " " << odl[v] << endl;
        if(odl[v] >= ret.st){
            ret.nd = max(ret.st, ret.nd);
            ret.st = odl[v];
        }

        for(auto i: graph[v]){
            if(odw[i] == true)continue;
            odw[i] = true;
            odl[i] = odl[v]+1;
//            cout << ">> " << i << "odl " << odl[i] << endl;
            que.push(i);
        }
    }
    return ret;
}

int main(){
    int n, m;
    scanf("%d%d", &n, &m);
    int it = mamm;
    vi prze;
    for(int i = 0; i < m; i++){
        int a, b;
        scanf("%d%d", &a, &b);
        graph[a].pb(b);
        graph[b].pb(a);
        it++;
    }
//        cout << bfs(1);
    int ans = 1e9;
    for(int i = 1; i <= n; i++){
        klar(odw);
        klar(odl);
//        cout << "check " << i << endl;
        pii h = bfs(i);
//        cout << "check " << i << " " << h.st << " " << h.nd << endl;
        ans = min(ans, h.st+h.nd);
    }
//    cout << ans << endl;
    printf("%d", ans/2);
    if((ans/2)*2 < ans)printf(".5");
    else printf(".0");
}