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#include <stdio.h> #include <string.h> #include <stdlib.h> #define M 1000000007 int add(int a, int b) { return (a + b) % M; } int mult(int a, int b) { return ((long long) a * b) % M; } #define K 20 struct edge { int v; struct edge *next; }; struct vertex { struct edge *list; int dp[K + 1][K + 2]; } *vv; int max(int a, int b, int k) { return a != k + 1 && b != k + 1 ? (a > b ? a : b) : (a == k + 1 ? b : a); } void edgeadd(int u, int v) { struct edge *x = malloc(sizeof(*x)); x->v = v; x->next = vv[u].list; vv[u].list = x; } void dfs(int u, int v, int n, int k) { int i, j, wi, wj, dp[K + 1][K + 2]; struct edge *x; for (x = vv[v].list; x != NULL; x = x->next) if (x->v != u) dfs(v, x->v, n, k); vv[v].dp[0][k + 1] = vv[v].dp[k][0] = 1; /* the jth node */ for (x = vv[v].list; x != NULL; x = x->next) { /* the tree */ int w = x->v; memset(dp, 0, (K + 1) * (K + 2) * sizeof(**dp)); if (w != u) { for (i = 0; i <= k; i++) for (j = 0; j <= k + 1; j++) if (vv[v].dp[i][j] != 0) for (wi = 0; wi <= k; wi++) for (wj = 0; wj <= k + 1; wj++) if (vv[w].dp[wi][wj] != 0) { int i_ = i < wi + 1 ? i : wi + 1; int j_ = k + 1; if (j != k + 1 && j + wi + 1 > k) j_ = max(j_, j, k); if (wj != k + 1 && wj + 1 + i > k) j_ = max(j_, wj + 1, k); if (j_ == k) continue; dp[i_][j_] = add(dp[i_][j_], mult(vv[v].dp[i][j], vv[w].dp[wi][wj])); } memcpy(vv[v].dp, dp, sizeof(**dp) * (K + 1) * (K + 2)); } } } int main() { int i, n, k, cnt; scanf("%d%d", &n, &k); vv = calloc(n, sizeof(*vv)); if (k == 0) printf("1\n"); else { for (i = 1; i < n; i++) { int u, v; scanf("%d%d", &u, &v); u--, v--; edgeadd(u, v); edgeadd(v, u); } dfs(-1, 0, n, k); cnt = 0; for (i = 0; i <= k; i++) cnt = add(cnt, vv[0].dp[i][k + 1]); printf("%d\n", cnt); } return 0; }
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