codeforces E. Famil Door and Roads 期望

//File Name: cf629E.cpp
  //Created Time: 2017年01月05日 星期四 17时20分29秒
                                   
#include <bits/stdc++.h>
#define LL long long
using namespace std;
const int MAXN = 100000 + 2;
int siz[MAXN],dis[MAXN];
int pa[MAXN][20];
LL f[MAXN],g[MAXN];
vector<int> G[MAXN];
void dfs0(int u,int p){
    siz[u] = 1;
    dis[u] = dis[p] + 1;
    f[u] = 0;
    for(int i=0;i<G[u].size();++i){
        int v = G[u][i];
        if(v == p) continue;
        pa[v][0] = u;
        dfs0(v,u);
        siz[u] += siz[v];
        f[u] += f[v] + siz[v];
    }
}
void dfs1(int u,int p,int n){
    for(int i=0;i<G[u].size();++i){
        int v = G[u][i];
        if(v == p) continue;
        g[v] = n + g[u] - 2 * siz[v];
        dfs1(v,u,n);
    }
}
void cal_pa(int n){
    for(int j=1;(1<<j)<=n;++j){
        for(int i=1;i<=n;++i){
            if(pa[i][j-1] != -1)
                pa[i][j] = pa[pa[i][j-1]][j-1];
        }
    }
}
int cal_lca(int a,int b){
    if(dis[a] < dis[b]) swap(a,b);
    int cnt = 0;
    for(;(1<<cnt)<=dis[a];++cnt);
    --cnt;
    for(int j=cnt;j>=0;--j){
        if(dis[a] - (1<<j) >= dis[b])
            a = pa[a][j];
    }
    if(a == b) return a;
    for(int j=cnt;j>=0;--j){
        if(pa[a][j] != -1 && pa[a][j] != pa[b][j])
            a = pa[a][j],b = pa[b][j];
    }
    return pa[a][0];
}
int cal(int u,int v){
    int cnt = 0;
    for(;(1<<cnt)<=dis[u];++cnt);
    --cnt;
    for(int j=cnt;j>=0;--j){
        if(dis[u] - (1<<j) > dis[v])
            u = pa[u][j];
    }
    return u;
}
void solve(int n,int m){
    memset(pa,-1,sizeof(pa));
    dfs0(1,0);   //dis,siz,in,f,pa[i][0],dep
    g[1] = f[1];
    dfs1(1,0,n);     //g
    cal_pa(n); // pa
    int u,v,lca,w;
    while(m--){
        scanf("%d %d",&u,&v);
        lca = cal_lca(u,v);
        if(lca != u && lca != v){
            int tmp = dis[u] + dis[v] - 2 * dis[lca] + 1;
            double ans = tmp + (f[u] + 0.0) / siz[u] + (f[v] + 0.0) / siz[v];
            printf("%.15f\n",ans);
        }
        else{
            if(lca == u) swap(u,v);
            w = cal(u,v);
            if(pa[w][0] != v){
                printf("-1");
                return ;
            }
            int tmp = dis[u] - dis[v] + 1;
            double ans = tmp + (f[u] + 0.0) / siz[u] + (g[v] - f[w] - siz[w] + 0.0) / (n - siz[w]);
            printf("%.15f\n",ans);
        }
    }
}
int main(){
    int n,m;
    scanf("%d %d",&n,&m);
    for(int i=1,u,v;i<n;++i){
        scanf("%d %d",&u,&v);
        G[u].push_back(v);
        G[v].push_back(u);
    }
    solve(n,m);
    return 0;
}