The Space War Solution

from collections import defaultdict
class Graph:
    def __init__(self):
        self.graph = defaultdict(list)
    def addEdge(self,u,v):
        self.graph[u].append(v)
    def BFS(self, s):
        path=[]
        d2={}
        queue = []
        queue.append(s)
        d2[s]=1

        while queue:
            s = queue.pop(0)
            path.append(s)
            for i in self.graph[s]:
                if i not in d2:
                    queue.append(i)
                    d2[i]=1
        return path
    def DIAMETER(self,s):
        path=[]
        d2={}
        dist={}
        queue=[]
        queue.append(s)
        d2[s]=1
        dist[s]=0
        d=0
        while queue:
            s= queue.pop()
            for i in self.graph[s]:
                if i not in d2:
                    queue.append(i)
                    dist[i]=dist[s]+1
                    d=max(dist[i],d)
                    d2[i]=1
        return d

g=Graph()
n,m=map(int,input().split())
d1={}
l1=list(range(1,n+1))
for item in l1:
    d1[item]=1
for i in range(0,m):
    x,y=map(int,input().split())
    g.addEdge(x,y)
    g.addEdge(y,x)
ans=0
for item in l1:
    if len(d1)==0:
        break
    else :
        if item in d1:

            path=g.BFS(item)
            d=g.DIAMETER(path[-1])
            ans=max(ans,(d+1)//2)
            for x in path:
               del d1[x]
print(ans)