CF804D Expected diameter of a tree 树的直径 根号分治

//#include<bits/stdc++.h>
#include<iostream>
#include<cstdio>
#include<ctime>
#include<cctype>
#include<queue>
#include<deque>
#include<stack>
#include<iostream>
#include<iomanip>
#include<cstdio>
#include<cstring>
#include<string>
#include<ctime>
#include<cmath>
#include<cctype>
#include<cstdlib>
#include<queue>
#include<deque>
#include<stack>
#include<vector>
#include<algorithm>
#include<utility>
#include<bitset>
#include<set>
#include<map>
#define ll long long
#define db double
#define INF 1000000001
#define ldb long double
#define pb push_back
#define put_(x) printf("%d ",x);
#define get(x) x=read()
#define gt(x) scanf("%d",&x)
#define gi(x) scanf("%lf",&x)
#define put(x) printf("%d\n",x)
#define putl(x) printf("%lld\n",x)
#define rep(p,n,i) for(RE int i=p;i<=n;++i)
#define go(x) for(int i=lin[x],tn=ver[i];i;tn=ver[i=nex[i]])
#define fep(n,p,i) for(RE int i=n;i>=p;--i)
#define vep(p,n,i) for(RE int i=p;i<n;++i)
#define pii pair<int,int>
#define mk make_pair
#define RE register
#define P 1000000007ll
#define gf(x) scanf("%lf",&x)
#define pf(x) ((x)*(x))
#define uint unsigned long long
#define ui unsigned
#define EPS 1e-4
#define sq sqrt
#define S second
#define F first
using namespace std;
char *fs,*ft,buf[1<<15];
inline char gc()
{
	return (fs==ft&&(ft=(fs=buf)+fread(buf,1,1<<15,stdin),fs==ft))?0:*fs++;
}
inline int read()
{
	RE int x=0,f=1;RE char ch=gc();
	while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=gc();}
	while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=gc();}
	return x*f;

}
const int MAXN=100010;
int n,m,Q,cnt,top,mx;
vector<ll>G[MAXN],c[MAXN];
int g[MAXN],f[MAXN],w[MAXN],s[MAXN],id[MAXN],vis[MAXN];
int lin[MAXN],ver[MAXN<<1],nex[MAXN<<1],len;
map<int,ll>H[MAXN];
inline void add(int x,int y)
{
	ver[++len]=y;nex[len]=lin[x];lin[x]=len;
	ver[++len]=x;nex[len]=lin[y];lin[y]=len;
}
inline void dfs(int x,int fa)
{
	vis[x]=cnt;
	go(x)if(tn!=fa)
	{
		dfs(tn,x);
		mx=max(mx,f[x]+f[tn]+1);
		if(f[tn]+1>f[x])
		{
			g[x]=f[x];
			f[x]=f[tn]+1;
			id[x]=tn;
		}
		else g[x]=max(g[x],f[tn]+1);
	}
}
inline void dp(int x,int fa)
{
	s[++top]=f[x];
	go(x)if(tn!=fa)
	{
		if(id[x]!=tn)
		{
			if(f[x]+1>f[tn])
			{
				g[tn]=f[tn];
				f[tn]=f[x]+1;
				id[tn]=x;
			}
			else g[tn]=max(g[tn],f[x]+1);
		}
		else
		{
			if(g[x]+1>f[tn])
			{
				g[tn]=f[tn];
				f[tn]=g[x]+1;
				id[tn]=x;
			}
			else g[tn]=max(g[tn],g[x]+1);
		}
		dp(tn,x);
	}
}
inline int find(int x,int y)
{
	
	int l=0,r=G[x].size()-1;
	while(l+1<r)
	{
		int mid=(l+r)>>1;
		if(c[x][mid]<=y)l=mid;
		else r=mid;
	}
	if(c[x][r]<=y)return r;
	return l;
}
inline ll calc(int x,int y)
{
	if(H[x].find(y)!=H[x].end())return H[x][y];
	int cc=max(w[x],w[y]);
	ll ans=0;
	vep(0,G[x].size(),i)
	{
		if(c[y][0]>cc-c[x][i]-1)
		{
			ans+=(ll)(1+c[x][i])*G[y].size()+G[y][G[y].size()-1];
			continue;
		}
		if(c[y][c[y].size()-1]<=cc-c[x][i]-1)
		{
			ans+=(ll)G[y].size()*cc;
			continue;
		}
		int ww=find(y,cc-c[x][i]-1);
		ans+=(ll)(ww+1)*cc+G[y][G[y].size()-1]-G[y][ww]+(1+c[x][i])*(G[y].size()-ww-1);
	}
	H[x][y]=ans;return ans;
}
int main()
{
	//freopen("1.in","r",stdin);
	get(n);get(m);get(Q);
	rep(1,m,i)add(read(),read());
	rep(1,n,i)
	{
		if(vis[i])continue;
		mx=top=0;++cnt;dfs(i,0);dp(i,0);
		sort(s+1,s+1+top);w[cnt]=mx;
		rep(1,top,j)G[cnt].pb(s[j]),c[cnt].pb(s[j]);
		rep(1,top-1,j)G[cnt][j]+=G[cnt][j-1];
	}
	rep(1,Q,i)
	{
		int get(x),get(y);
		if(vis[x]==vis[y]){puts("-1");continue;}
		x=vis[x];y=vis[y];
		if(G[x].size()>G[y].size())swap(x,y);
		if(G[x].size()==G[y].size()&&x>y)swap(x,y);
		printf("%.7lf\n",1.0*calc(x,y)/(G[x].size()*G[y].size()));
	}
	return 0;
}