#include <bits/stdc++.h>
using namespace std;

typedef long long   signed int LL;
typedef long long unsigned int LU;

#define incID(i, l, r) for(int i = (l)    ; i <  (r); i++)
#define incII(i, l, r) for(int i = (l)    ; i <= (r); i++)
#define decID(i, l, r) for(int i = (r) - 1; i >= (l); i--)
#define decII(i, l, r) for(int i = (r)    ; i >= (l); i--)
#define  inc(i, n) incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define  dec(i, n) decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)

#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define inID(v, l, r) ((l) <= (v) && (v) <  (r))

#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define PQ priority_queue

#define  ALL(v)  v.begin(),  v.end()
#define RALL(v) v.rbegin(), v.rend()
#define  FOR(it, v) for(auto it =  v.begin(); it !=  v.end(); ++it)
#define RFOR(it, v) for(auto it = v.rbegin(); it != v.rend(); ++it)

template<typename T> bool   setmin(T & a, T b) { if(b <  a) { a = b; return true; } else { return false; } }
template<typename T> bool   setmax(T & a, T b) { if(b >  a) { a = b; return true; } else { return false; } }
template<typename T> bool setmineq(T & a, T b) { if(b <= a) { a = b; return true; } else { return false; } }
template<typename T> bool setmaxeq(T & a, T b) { if(b >= a) { a = b; return true; } else { return false; } }
template<typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template<typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

// ---- ----

LL n, k, c[2000], dp[2000][2001], MOD = 1e9 + 7;
vector<int> g[2000];

void dfs(int v) {
	c[v] = 1;
	for(auto && e: g[v]) { dfs(e); c[v] += c[e]; }
	
	dp[v][0] = 1;
	for(auto && e: g[v]) {
		decII(i, 0, c[v]) { if(dp[v][i] == 0) { continue; }
		decII(j, 1, c[e]) { if(dp[e][j] == 0) { continue; }
			(dp[v][i + j] += dp[v][i] * dp[e][j]) %= MOD;
		}
		}
	}
	dp[v][c[v]] = 1;
}

int main() {
	cin >> n >> k;
	inc(i, n - 1) {
		int a, b;
		cin >> a >> b;
		g[a].PB(b);
	}
	
	dfs(0);
	
	cout << dp[0][k] << endl;
	
	return 0;
}