#pragma GCC optimize ("Ofast")
#include<bits/stdc++.h>
using namespace std;
#define MD (1000000007U)
void*wmem;
char memarr[96000000];
template<class S, class T> inline S max_L(S a,T b){
  return a>=b?a:b;
}
template<class T> inline void walloc1d(T **arr, int x, void **mem = &wmem){
  static int skip[16] = {0, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1};
  (*mem) = (void*)( ((char*)(*mem)) + skip[((unsigned long long)(*mem)) & 15] );
  (*arr)=(T*)(*mem);
  (*mem)=((*arr)+x);
}
struct Modint{
  unsigned val;
  Modint(){
    val=0;
  }
  Modint(int a){
    val = ord(a);
  }
  Modint(unsigned a){
    val = ord(a);
  }
  Modint(long long a){
    val = ord(a);
  }
  Modint(unsigned long long a){
    val = ord(a);
  }
  inline unsigned ord(unsigned a){
    return a%MD;
  }
  inline unsigned ord(int a){
    a %= (int)MD;
    if(a < 0){
      a += MD;
    }
    return a;
  }
  inline unsigned ord(unsigned long long a){
    return a%MD;
  }
  inline unsigned ord(long long a){
    a %= (int)MD;
    if(a < 0){
      a += MD;
    }
    return a;
  }
  inline unsigned get(){
    return val;
  }
  inline Modint &operator+=(Modint a){
    val += a.val;
    if(val >= MD){
      val -= MD;
    }
    return *this;
  }
  inline Modint &operator-=(Modint a){
    if(val < a.val){
      val = val + MD - a.val;
    }
    else{
      val -= a.val;
    }
    return *this;
  }
  inline Modint &operator*=(Modint a){
    val = ((unsigned long long)val*a.val)%MD;
    return *this;
  }
  inline Modint &operator/=(Modint a){
    return *this *= a.inverse();
  }
  inline Modint operator+(Modint a){
    return Modint(*this)+=a;
  }
  inline Modint operator-(Modint a){
    return Modint(*this)-=a;
  }
  inline Modint operator*(Modint a){
    return Modint(*this)*=a;
  }
  inline Modint operator/(Modint a){
    return Modint(*this)/=a;
  }
  inline Modint operator+(int a){
    return Modint(*this)+=Modint(a);
  }
  inline Modint operator-(int a){
    return Modint(*this)-=Modint(a);
  }
  inline Modint operator*(int a){
    return Modint(*this)*=Modint(a);
  }
  inline Modint operator/(int a){
    return Modint(*this)/=Modint(a);
  }
  inline Modint operator+(long long a){
    return Modint(*this)+=Modint(a);
  }
  inline Modint operator-(long long a){
    return Modint(*this)-=Modint(a);
  }
  inline Modint operator*(long long a){
    return Modint(*this)*=Modint(a);
  }
  inline Modint operator/(long long a){
    return Modint(*this)/=Modint(a);
  }
  inline Modint operator-(void){
    Modint res;
    if(val){
      res.val=MD-val;
    }
    else{
      res.val=0;
    }
    return res;
  }
  inline operator bool(void){
    return val!=0;
  }
  inline operator int(void){
    return get();
  }
  inline operator long long(void){
    return get();
  }
  inline Modint inverse(){
    int a = val;
    int b = MD;
    int u = 1;
    int v = 0;
    int t;
    Modint res;
    while(b){
      t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    if(u < 0){
      u += MD;
    }
    res.val = u;
    return res;
  }
  inline Modint pw(unsigned long long b){
    Modint a(*this);
    Modint res;
    res.val = 1;
    while(b){
      if(b&1){
        res *= a;
      }
      b >>= 1;
      a *= a;
    }
    return res;
  }
  inline bool operator==(int a){
    return ord(a)==val;
  }
  inline bool operator!=(int a){
    return ord(a)!=val;
  }
}
;
inline Modint operator+(int a, Modint b){
  return Modint(a)+=b;
}
inline Modint operator-(int a, Modint b){
  return Modint(a)-=b;
}
inline Modint operator*(int a, Modint b){
  return Modint(a)*=b;
}
inline Modint operator/(int a, Modint b){
  return Modint(a)/=b;
}
inline Modint operator+(long long a, Modint b){
  return Modint(a)+=b;
}
inline Modint operator-(long long a, Modint b){
  return Modint(a)-=b;
}
inline Modint operator*(long long a, Modint b){
  return Modint(a)*=b;
}
inline Modint operator/(long long a, Modint b){
  return Modint(a)/=b;
}
inline int my_getchar_unlocked(){
  static char buf[1048576];
  static int s = 1048576;
  static int e = 1048576;
  if(s == e && e == 1048576){
    e = fread_unlocked(buf, 1, 1048576, stdin);
    s = 0;
  }
  if(s == e){
    return EOF;
  }
  return buf[s++];
}
inline void rd(int &x){
  int k;
  int m=0;
  x=0;
  for(;;){
    k = my_getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = my_getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
struct MY_WRITER{
  char buf[1048576];
  int s;
  int e;
  MY_WRITER(){
    s = 0;
    e = 1048576;
  }
  ~MY_WRITER(){
    if(s){
      fwrite_unlocked(buf, 1, s, stdout);
    }
  }
}
;
MY_WRITER MY_WRITER_VAR;
void my_putchar_unlocked(int a){
  if(MY_WRITER_VAR.s == MY_WRITER_VAR.e){
    fwrite_unlocked(MY_WRITER_VAR.buf, 1, MY_WRITER_VAR.s, stdout);
    MY_WRITER_VAR.s = 0;
  }
  MY_WRITER_VAR.buf[MY_WRITER_VAR.s++] = a;
}
inline void wt_L(char a){
  my_putchar_unlocked(a);
}
inline void wt_L(int x){
  int s=0;
  int m=0;
  char f[10];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar_unlocked('-');
  }
  while(s--){
    my_putchar_unlocked(f[s]+'0');
  }
}
inline void wt_L(Modint x){
  int i;
  i = (int)x;
  wt_L(i);
}
struct graph{
  int N;
  int*es;
  int**edge;
  void setEdge(int N__, int M, int A[], int B[], void **mem = &wmem){
    int i;
    N = N__;
    walloc1d(&es, N, mem);
    walloc1d(&edge, N, mem);
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      es[A[i]]++;
      es[B[i]]++;
    }
    for(i=(0);i<(N);i++){
      walloc1d(&edge[i], es[i], mem);
    }
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      edge[A[i]][es[A[i]]++] = B[i];
      edge[B[i]][es[B[i]]++] = A[i];
    }
  }
  void setEdgeRootedTree(int N__, int M, int A[], int B[], int root=0, int reorder=0, int cnv[] = NULL, void **mem = &wmem){
    int i;
    int j;
    int k;
    int*dist;
    int*q;
    int qs;
    int qe;
    int*ind;
    void*tmem;
    N = N__;
    tmem = ((char*)(*mem)) + (sizeof(int) * N + 15) + (sizeof(int*) * N + 15) + (sizeof(int) * M + 15 * N);
    walloc1d(&es, N, mem);
    walloc1d(&edge, N, mem);
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      es[A[i]]++;
      es[B[i]]++;
    }
    for(i=(0);i<(N);i++){
      walloc1d(&edge[i], es[i], &tmem);
    }
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      edge[A[i]][es[A[i]]++] = B[i];
      edge[B[i]][es[B[i]]++] = A[i];
    }
    walloc1d(&dist, N, &tmem);
    walloc1d(&q, N, &tmem);
    walloc1d(&ind, N, &tmem);
    if(cnv==NULL){
      walloc1d(&cnv, N, &tmem);
    }
    for(i=(0);i<(N);i++){
      dist[i] = -1;
    }
    dist[root] = 0;
    qs = qe = 0;
    q[qe++] = root;
    while(qs < qe){
      i = q[qs++];
      for(j=(0);j<(es[i]);j++){
        k = edge[i][j];
        if(dist[k]==-1){
          dist[k] = dist[i] + 1;
          q[qe++] = k;
        }
      }
    }
    if(reorder == 0){
      for(i=(0);i<(N);i++){
        cnv[i] = i;
      }
      for(i=(0);i<(N);i++){
        ind[i] = i;
      }
    }
    else{
      for(i=(0);i<(N);i++){
        cnv[i] = q[i];
      }
      for(i=(0);i<(N);i++){
        ind[cnv[i]] = i;
      }
    }
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      j = A[i];
      k = B[i];
      if(dist[j] > dist[k]){
        swap(j, k);
      }
      es[ind[j]]++;
    }
    for(i=(0);i<(N);i++){
      walloc1d(&edge[i], es[i], mem);
    }
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      j = A[i];
      k = B[i];
      if(dist[j] > dist[k]){
        swap(j, k);
      }
      j = ind[j];
      k = ind[k];
      edge[j][es[j]++] = k;
    }
  }
  void getDist(int root, int res[], void *mem = wmem){
    int i;
    int j;
    int k;
    int*q;
    int s;
    int z;
    walloc1d(&q, N, &mem);
    for(i=(0);i<(N);i++){
      res[i]=-1;
    }
    res[root]=0;
    s=0;
    z=1;
    q[0]=root;
    while(z){
      i=q[s++];
      z--;
      for(j=(0);j<(es[i]);j++){
        k=edge[i][j];
        if(res[k]>=0){
          continue;
        }
        res[k]=res[i]+1;
        q[s+z++]=k;
      }
    }
  }
}
;
int N;
int A[1000];
int B[1000];
int C;
graph g;
int dist[1001];
int dmax[1001];
Modint memo[1001];
Modint dp[1000][1001];
int main(){
  int i, r;
  wmem = memarr;
  Modint res = 0;
  Modint tmp;
  rd(N);
  rd(C);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=(0);Lj4PdHRW<(N-1);Lj4PdHRW++){
      rd(A[Lj4PdHRW]);A[Lj4PdHRW] += (-1);
      rd(B[Lj4PdHRW]);B[Lj4PdHRW] += (-1);
    }
  }
  g.setEdge(N,N-1,A,B);
  for(i=(0);i<(N);i++){
    g.getDist(i,dist);
    {
      int RZTsC2BF;
      int FmcKpFmN;
      if(N==0){
        FmcKpFmN = 0;
      }
      else{
        FmcKpFmN = dist[0];
        for(RZTsC2BF=(1);RZTsC2BF<(N);RZTsC2BF++){
          FmcKpFmN = max_L(FmcKpFmN, dist[RZTsC2BF]);
        }
      }
      dmax[i] =FmcKpFmN;
    }
  }
  {
    int KrdatlYV;
    int ao_dF3pO = 0;
    int tU__gIr_;
    int a2conNHc;
    int hCmBdyQB;
    for(KrdatlYV=(0);KrdatlYV<(((N)-1)+1);KrdatlYV++){
      a2conNHc = dmax[KrdatlYV];
      if(ao_dF3pO==0 || tU__gIr_>a2conNHc){
        tU__gIr_ = a2conNHc;
        ao_dF3pO = 1;
        hCmBdyQB = KrdatlYV;
      }
    }
    g.setEdgeRootedTree(N,N-1,A,B,hCmBdyQB,1);
  }
  for(r=(2);r<(N+1);r++){
    int j, k;
    for(i=(N)-1;i>=(0);i--){
      int j;
      for(j=(0);j<(r);j++){
        int XJIcIBrW;
        tmp = 1;
        for(XJIcIBrW=(0);XJIcIBrW<(g.es[i]);XJIcIBrW++){
          auto &k = g.edge[i][XJIcIBrW];
          if(j-1 >= 0){
            if(j+1 < r){
              tmp *=dp[k][j-1]+dp[k][j+1];
            }
            else{
              tmp *=dp[k][j-1]+0;
            }
          }
          else{
            if(j+1 < r){
              tmp *=0+dp[k][j+1];
            }
            else{
              tmp *=0+0;
            }
          }
        }
        dp[i][j] = tmp;
      }
    }
    for(j=(0);j<(r);j++){
      memo[r] += dp[0][j];
    }
    for(j=(1);j<(r);j++){
      memo[r] -= memo[j] * (r-j+1);
    }
    if(memo[r]==0){
      break;
    }
    for(k=(0);k<(3);k++){
      i = (C + k) / 3;
      if(i >= r){
        res += memo[r] * (i-r+1);
      }
    }
  }
  wt_L(res);
  wt_L('\n');
  return 0;
}
// cLay varsion 20200911-1

// --- original code ---
// int N, A[1000], B[1000], C;
// graph g;
// 
// int dist[1001], dmax[1001];
// Modint memo[1001];
// Modint dp[1000][1001];
// 
// {
// 
//   Modint res = 0, tmp;
//   rd(N,C,(A--,B--)(N-1));
// 
//   g.setEdge(N,N-1,A,B);
//   rep(i,N) g.getDist(i,dist), dmax[i] = max(dist(N));
//   g.setEdgeRootedTree(N,N-1,A,B,argmin(dmax(N)),1);
// 
//   rep(r,2,N+1){
//     rrep(i,N) rep(j,r){
//       tmp = 1;
//       rep[g.edge[i]](k,g.es[i]){
// //        wt("edge",i,k);
//         tmp *= if[j-1 >= 0, dp[k][j-1], 0] + if[j+1 < r, dp[k][j+1], 0];
//       }
//       dp[i][j] = tmp;
// //      wt(r,i,j,tmp);
//     }
//     rep(j,r) memo[r] += dp[0][j];
//     rep(j,1,r) memo[r] -= memo[j] * (r-j+1);
//     if(memo[r]==0) break;
// //    wt("r",r,memo[r]);
// 
//     rep(k,3){
//       i = (C + k) / 3;
//       if(i >= r) res += memo[r] * (i-r+1);
//     }
//   }
// 
//   wt(res);
// }