The Settlers of Catan

#include<iostream>
#include<cstdio>
#include<cstring>
#define MAXN 30
using namespace std;
int n,m, G[MAXN][MAXN], vis[MAXN][MAXN], maxNum;
 
 
void dfs(int u, int num)
{
    for(int v=0; v<n; ++v)
    {
        if(G[u][v] && !vis[u][v])
        {
            vis[u][v] = vis[v][u] = 1;
            dfs(v, num+1);
            vis[u][v] = vis[v][u] = 0;
        }
    }
    if(num > maxNum) maxNum = num;
}
 
 
int main()
{
    

    int a,b;
    while(~scanf("%d %d", &n, &m)&&n&&m)
    {
        memset(G, 0, sizeof(G));
        for(int i=0; i<m; ++i)
        {
            scanf("%d %d", &a, &b);
            G[a][b]++;  
            G[b][a]++;
        }
        
        maxNum = 0;
        for(int i=0; i<n; ++i)
        {
            memset(vis, 0, sizeof(vis));
            dfs(i, 0);
        }
        printf("%d\n",maxNum);
    } 
    return 0;
}