Problem:
Score : 600 points

Problem Statement

Given is a tree T with N vertices. The i-th edge connects Vertex A_i and B_i (1 \leq A_i,B_i \leq N).

Now, each vertex is painted black with probability 1/2 and white with probability 1/2, which is chosen independently from other vertices. Then, let S be the smallest subtree (connected subgraph) of T containing all the vertices painted black. (If no vertex is painted black, S is the empty graph.)

Let the holeyness of S be the number of white vertices contained in S. Find the expected holeyness of S.

Since the answer is a rational number, we ask you to print it \bmod 10^9+7, as described in Notes.

Notes

When you print a rational number, first write it as a fraction \frac{y}{x}, where x, y are integers, and x is not divisible by 10^9 + 7
(under the constraints of the problem, such representation is always possible).

Then, you need to print the only integer z between 0 and 10^9 + 6, inclusive, that satisfies xz \equiv y \pmod{10^9 + 7}.

Constraints

2 \leq N \leq 2 \times 10^5

1 \leq A_i,B_i \leq N

The given graph is a tree.

Input

Input is given from Standard Input in the following format:

N
A_1 B_1
:
A_{N-1} B_{N-1}

Output

Print the expected holeyness of S, \bmod 10^9+7.

Sample Input 1

3
1 2
2 3

Sample Output 1

125000001

If the vertices 1, 2, 3 are painted black, white, black, respectively, the holeyness of S is 1.

Otherwise, the holeyness is 0, so the expected holeyness is 1/8.

Since 8 \times 125000001 \equiv 1 \pmod{10^9+7}, we should print 125000001.

Sample Input 2

4
1 2
2 3
3 4

Sample Output 2

375000003

The expected holeyness is 3/8.

Since 8 \times 375000003 \equiv 3 \pmod{10^9+7}, we should print 375000003.

Sample Input 3

4
1 2
1 3
1 4

Sample Output 3

250000002

The expected holeyness is 1/4.

Sample Input 4

7
4 7
3 1
2 6
5 2
7 1
2 7

Sample Output 4

570312505

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;

// template {{{  0 
// using {{{ 1
using ll = long long int;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vii = vector<pii>;
using vll = vector<pll>;
// }}} 1
// definition {{{ 1
// scaning {{{ 2
#define Scd(x) scanf("%d", &x)
#define Scd2(x,y) scanf("%d%d", &x, &y)
#define Scd3(x,y,z) scanf("%d%d%d", &x, &y, &z)
#define Scll(x) scanf("%lld", &x)
#define Scll2(x,y) scanf("%lld%lld", &x, &y)
#define Scll3(x,y,z) scanf("%lld%lld%lld", &x, &y, &z)
#define Scc(c) scanf("%c", &c);
#define Scs(s) scanf("%s", s);
#define Scstr(s) scanf("%s", &s);
// }}} 2
// constants {{{ 2
#define EPS (1e-7)
#define INF (2e9)
#define PI (acos(-1))
// }}} 2
// systems {{{ 2
#define Repe(x,y,z) for(ll x = z; x < y; x++)
#define Rep(x,y) Repe(x,y,0)
#define RRepe(x,y,z) for(ll x = y-z-1; x >= 0; x--)
#define RRep(x,y) RRepe(x,y,0)
// }}} 2
// output {{{ 2
#define YesNo(a) (a)?printf("Yes\n"):printf("No\n")
#define YESNO(a) (a)?printf("YES\n"):printf("NO\n")
// }}} 2
// }}} 1
// input {{{ 1
// }}} 1
// }}} 0

const long long int mod = 1000000007;

// PowMod( base, index, modulo) return base ** index % modulo {{{
// PowMod = base ** index % mod ( natural numbers )
inline constexpr long long int PowMod( long long int base, long long int index, long long int modulo = mod ){
    if( index == 0 ) return 1;
    // O( log(index) )
    if( index % 2 ){
        return base * PowMod(base, index - 1, modulo) % modulo;
    }else{
        long long int Phalf = index / 2;
        long long int half = PowMod(base, Phalf, modulo);
        return half * half % modulo;
    }
}
// }}}
// CombMod( n, r, modulo ) return nCr % modulo {{{
// CombMod(ination) = nCr % mod ( natural number )
inline constexpr long long int CombMod( long long int n, long long int r, long long int modulo = mod){
    if( n < r ) return CombMod(r,n,modulo);
    long long int Upper = 1;
    long long int Lower = 1;
    for(long long int i = 0; i < r; i++){
        Upper = Upper * (n-i) % modulo;
        Lower = Lower * (i+1) % modulo;
    }
    // Return (Upper / Lower)
    long long int ret = Upper * PowMod(Lower,modulo-2,modulo) % modulo;
    return ret;
}
// }}}
// FactMod( n, modulo ) return n! % modulo {{{
// Fact(orial) = n! % mod ( natural number )
inline constexpr long long int Fact( long long int n, long long int modulo = mod){
    long long int Upper = 1;
    for(long long int i = 0; i < n; i++){
        Upper = Upper * (n-i) % modulo;
    }
    return Upper;
}
// }}}
// modint {{{

struct modint{
    long long int a;
    inline constexpr modint( long long int x = 0 ) noexcept : a(x % mod) {}
    inline constexpr long long int &value() noexcept { return a; }
    inline constexpr const long long int &value() const noexcept { return a; }
    inline constexpr modint operator+(const modint rhs) const noexcept{ return modint(*this) += rhs; }
    inline constexpr modint operator+(const int rhs) const noexcept{ return modint(*this) += rhs; }
    inline constexpr modint operator+(const long long int rhs) const noexcept{ return modint(*this) += rhs; }
    inline constexpr modint operator-(const modint rhs) const noexcept{ return modint(*this) -= rhs; }
    inline constexpr modint operator-(const int rhs) const noexcept{ return modint(*this) -= rhs; }
    inline constexpr modint operator-(const long long int rhs) const noexcept{ return modint(*this) -= rhs; }
    inline constexpr modint operator*(const modint rhs) const noexcept{ return modint(*this) *= rhs; }
    inline constexpr modint operator*(const int rhs) const noexcept{ return modint(*this) *= rhs; }
    inline constexpr modint operator*(const long long int rhs) const noexcept{ return modint(*this) *= rhs; }
    inline constexpr modint operator/(const modint rhs) const noexcept{ return modint(*this) /= rhs; }
    inline constexpr modint operator/(const int rhs) const noexcept{ return modint(*this) /= rhs; }
    inline constexpr modint operator/(const long long int rhs) const noexcept{ return modint(*this) /= rhs; }
    inline constexpr modint operator+=(const modint rhs) noexcept{ a += rhs.a; if( a >= mod ) a -= mod; return *this; }
    inline constexpr modint operator-=(const modint rhs) noexcept{ a -= rhs.a; if( a < 0 ) a += mod; return *this; }
    inline constexpr modint operator*=(const modint rhs) noexcept{ a = a * rhs.a % mod ;  return *this; }
    inline constexpr modint operator/=(const modint rhs) noexcept{ a = a * PowMod(rhs.a,mod-2) % mod; return *this; }
};

// }}}

vector<vi> e;
vi subN;
vector<vi> d;
vector<modint> ans;

void dfs1( int from, int pare ){
    subN[from] = 1;
    for( int to : e[from] ){
        if( to == pare ) continue;
        dfs1(to, from);
        subN[from] += subN[to];
        d[from].push_back( subN[to] );
    }
}

void dfs2( int from, int pare, int dpare ){
    int cnt = dpare;
    // ans
    for( int dN : d[from] ){
        cnt += dN;
    }
    ans[from] += modint(1)/PowMod(2,cnt); // all white
    // one black other white
    ans[from] += modint(1)*(PowMod(2,dpare)-1)/PowMod(2,cnt);
    for( int dN : d[from] ){
        ans[from] += modint(1)*(PowMod(2,dN)-1)/PowMod(2,cnt);
    }
    ans[from] = modint(1) - ans[from];

    // dfs
    cnt = 0;
    for( int to : e[from] ){
        if( to == pare ) continue;
        dfs2(to, from, dpare+subN[from]-d[from][cnt++] );
    }
}

int main() {

    int N;
    Scd(N);
    e.resize(N);
    int x,y;
    Rep(i,N-1){
        Scd2(x,y);
        x--,y--;
        e[x].push_back(y);
        e[y].push_back(x);
    }
    subN.assign(N,0);
    ans.assign(N,modint(0));
    d.resize(N);

    dfs1(0,-1);

    Rep(i,N){
//        for( int c : d[i] ) printf ("%d:", c );
//        printf ("\n");
    }

    dfs2(0,-1,0);

    modint answ = 0;
    Rep(i,N){
        answ += ans[i];
    }

    printf ("%lld\n", answ/2 );

    return 0;
}