#include <bits/stdc++.h>   using namespace std;   using LL = long long;   namespace _buff {   const size_t BUFF = 1 << 19;   char buff[BUFF], *begin = buff, *end = buff;   char getc() {     if (begin == end) {       begin = buff;       end = buff + fread(buff, 1, BUFF, stdin);     }     return begin == end ? -1 : *begin++;   }   }  // namespace _buff   LL read() {     using namespace _buff;     LL ret = 0;     bool pos = true;     char c = getc();     for (; (c <  0  || c >  9 ) && c !=  - ; c = getc()) {       assert(~c);     }     if (c ==  - ) {       pos = false;       c = getc();     }     for (; c >=  0  && c <=  9 ; c = getc()) {       ret = (ret << 3) + (ret << 1) + (c ^ 48);     }     return pos ? ret : -ret;   }   const size_t N = 1 << 19;   const int MOD = 998244353;   const int ROOT = 3;   int qpow(int base, int e) {     int ret = 1;     for (; e; e >>= 1) {       if (e & 1) ret = 1LL * ret * base % MOD;       base = 1LL * base * base % MOD;     }     return ret;   }   int rmod(int x) {     if (x >= MOD) x -= MOD;     if (x < 0) x += MOD;     return x;   }   void fft(int a[], int lg, int flag) {     static int rev[N], rev_lg = -1;     int n = 1 << lg;     if (lg != rev_lg) {       for (int i = 0; i < n; ++i) {         rev[i] = rev[i >> 1] >> 1 | ((i & 1) << (lg - 1));       }     }     for (int i = 0; i < n; ++i) {       if (i < rev[i]) swap(a[i], a[rev[i]]);     }     for (int m = 1, l = 2; m < n; m <<= 1, l <<= 1) {       int wn = qpow(ROOT, flag ? (MOD - 1) - (MOD - 1) / l : (MOD - 1) / l);       for (int i = 0, j, w, x, y; i < n; i += l) {         for (j = 0, w = 1; j < m; ++j, w = 1LL * w * wn % MOD) {           x = a[i + j];           y = 1LL * w * a[i + j + m] % MOD;           a[i + j] = rmod(x + y);           a[i + j + m] = rmod(x - y);         }       }     }     if (flag) {       int inv = qpow(n, MOD - 2);       for (int i = 0; i < n; ++i) {         a[i] = 1LL * a[i] * inv % MOD;       }     }   }   vector<int> operator*(const vector<int> &a, const vector<int> &b) {     static int aa[N], bb[N], cc[N];     int n = a.size(), m = b.size(), l = n + m - 1, lg = 0;     vector<int> ans(l);     if (static_cast<LL>(n) * m <= 4096) {       static unsigned long long tmp[N];       for (int i = 0; i < n; ++i) {         for (int j = 0; j < m; ++j) {           if ((tmp[i + j] += static_cast<unsigned long long>(a[i]) * b[j]) >>               62) {             tmp[i + j] %= MOD;           }         }       }       for (int i = 0; i < l; ++i) {         ans[i] = tmp[i] % MOD;         tmp[i] = 0;       }       return ans;     }     for (; (1 << lg) < l; ++lg) {     }     memset(aa, 0, sizeof(int) << lg);     copy(a.begin(), a.end(), aa);     fft(aa, lg, 0);     memset(bb, 0, sizeof(int) << lg);     copy(b.begin(), b.end(), bb);     fft(bb, lg, 0);     for (int i = 0, _ = 1 << lg; i < _; ++i) {       cc[i] = 1LL * aa[i] * bb[i] % MOD;     }     fft(cc, lg, 1);     copy(cc, cc + l, ans.begin());     return ans;   }   vector<int> operator+(const vector<int> &a, const vector<int> &b) {     const vector<int> &aa = a.size() > b.size() ? a : b;     const vector<int> &bb = a.size() > b.size() ? b : a;     vector<int> ret(aa);     auto it = ret.begin();     auto jt = bb.begin();     for (; jt != bb.end(); ++it, ++jt) {       *it = rmod(*it + *jt);     }     return ret;   }   vector<int> g[N];   int pa[N], dep[N], nxt[N], sz[N];   void dfs1(int u) {     sz[u] = 1;     for (int v : g[u]) {       if (v == pa[u]) continue;       pa[v] = u;       dep[v] = dep[u] + 1;       dfs1(v);       sz[u] += sz[v];       if (sz[v] > sz[nxt[u]]) nxt[u] = v;     }   }   vector<int> polys[N];   pair<vector<int>, vector<int> > solve(const vector<int> &idx, int l, int r) {     if (l + 1 == r) {       return {polys[idx[l]], polys[idx[l]]};     }     int mid = (l + r) >> 1;     auto ans1 = solve(idx, l, mid), ans2 = solve(idx, mid, r);     return {ans1.first + ans2.first * ans1.second, ans1.second * ans2.second};   }   void dfs2(int u) {     assert(!pa[u] || nxt[pa[u]] != u);     vector<int> idx(1, u);     for (int v = nxt[u]; v; v = nxt[v]) {       idx.emplace_back(v);     }     for (int u : idx) {       polys[u] = {0, 1};       for (int v : g[u]) {         if (v == pa[u] || v == nxt[u]) continue;         dfs2(v);         polys[v][0] += 1;         polys[u] = polys[u] * polys[v];       }     }     polys[u] = solve(idx, 0, idx.size()).first;   }   int main() {     int n = read();     LL x = read();     for (int i = 1, a, b; i < n; ++i) {       a = read();       b = read();       g[a].emplace_back(b);       g[b].emplace_back(a);     }     dfs1(1);     dfs2(1);     vector<int> &vc = polys[1];     int ans = 0, mul = 1;     for (unsigned i = 1; i < vc.size(); ++i) {       ans = (ans + 1LL * mul * vc[i]) % MOD;       mul = (x + i) % MOD * qpow(i, MOD - 2) % MOD * mul % MOD;     }     printf( %d n , ans);     return 0;   }