tairos

#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
ifstream fin("tairos.in");
ofstream fout("tairos.out");

#define MOD 1000000007
#define ll long long

vector<ll> ADJ[1005];

ll N,D,rez,nivel;

bool VIZ[1005];

ll LEVEL[1000005],LEVEL_F[1000005],dp[1000005];

void df (ll parinte,ll lev)
{
    nivel=max(nivel,lev);
    vector<ll> :: iterator it;
    bool semafor=0;
    for (it=ADJ[parinte].begin();it!=ADJ[parinte].end();++it)
    {
        ll fiu=(*it);
        if (VIZ[fiu]==0)
        {
            VIZ[fiu]=1;
            semafor=1;
            df(fiu,lev+1);
        }
    }
    if (semafor==0)
    {
        ++LEVEL_F[lev];
    }
    else
    {
        ++LEVEL[lev];
    }
}

int main()
{
    int i,j;
    fin >> N >> D;
    int x,y;
    while (fin >> x >> y)
    {
        ADJ[x].push_back(y);
        ADJ[y].push_back(x);
    }
    VIZ[1]=1;
    df(1,0);
    dp[0]=1;
    for (i=0;i<=D;++i)
    {
        for (j=1;j<=nivel;++j)
        {
            dp[i+j]+=(dp[i]*LEVEL_F[j])%MOD;
        }
    }
    for (i=0;i<=D;++i)
    {
        rez+=(dp[D-i]*LEVEL[i])%MOD;
    }
    fout << rez%MOD;
    fin.close();
    fout.close();
    return 0;
}