#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000

struct combi{
	deque<mint> kaijou;
	deque<mint> kaijou_;
	
	combi(int n){
		kaijou.push_back(1);
		for(int i=1;i<=n;i++){
			kaijou.push_back(kaijou[i-1]*i);
		}
		
		mint b=kaijou[n].inv();
		
		kaijou_.push_front(b);
		for(int i=1;i<=n;i++){
			int k=n+1-i;
			kaijou_.push_front(kaijou_[0]*k);
		}
	}
	
	mint combination(int n,int r){
		if(r>n)return 0;
		mint a = kaijou[n]*kaijou_[r];
		a *= kaijou_[n-r];
		return a;
	}
	
	mint junretsu(int a,int b){
		mint x = kaijou_[a]*kaijou_[b];
		x *= kaijou[a+b];
		return x;
	}
	
	mint catalan(int n){
		return combination(2*n,n)/(n+1);
	}
	
};
combi Co(500000);

vector<vector<int>> E;
mint c;
vector<mint> dp;
mint INV;
void dfs(int cur,int p){
	rep(i,E[cur].size()){
		int to = E[cur][i];
		if(to==p)continue;
		dfs(to,cur);
		dp[cur] *= dp[to];
	}
	int d = E[cur].size();
	
	mint temp = 0;
	{
		mint X = 1;
		rep(i,E[cur].size())X *= c;
		temp += X;
	}
	
	{
		mint Y = 0;
		mint Temp = 1;
		rep(i,E[cur].size()){
			int remain = d-i;
			if(remain > d/2){
				
				Y += Co.combination(d,i) * Temp;
				
			}
			else break;
			Temp *= c-1;
		}
		Y *= c;
		
		temp -= Y;
	}
	temp += c;
	if(cur!=0)temp *= INV;
	dp[cur] *= temp;
}
	
	

int main(){
	
	int N,C;
	cin>>N>>C;
	
	E.resize(N);
	
	rep(i,N-1){
		int u,v;
		scanf("%d %d",&u,&v);
		u--;v--;
		E[u].push_back(v);
		E[v].push_back(u);
	}
	mint ans = 0;
	rep(i,C){
		c = C - i;
		mint temp = Co.combination(C,C-i);
		INV = c.inv();
		dp.assign(N,1);
		dfs(0,-1);
		
		
		temp *= dp[0];
		//cout<<temp.val()<<endl;
		if(i%2==1)temp *= -1;
		ans += temp;
	}
	
	cout<<ans.val()<<endl;

	return 0;
}