这是一道板子题

#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
int u[50010<<1],v[50010<<1],fir[50010],nxt[50010<<1],cnt,root,sz[50010],f[50010],middis[50010],midcnt,n,k,vis[50010],Siz;
long long ans=0;
void addedge(int ui,int vi){++cnt;u[cnt]=ui;v[cnt]=vi;nxt[cnt]=fir[ui];fir[ui]=cnt;
}
void getroot(int u,int fa){sz[u]=1,f[u]=1;for(int i=fir[u];i;i=nxt[i]){if(v[i]==fa||vis[v[i]])continue;getroot(v[i],u);sz[u]+=sz[v[i]];f[u]=max(f[u],sz[v[i]]);}f[u]=max(Siz-sz[u],f[u]);if(f[u]<f[root])root=u;
}
void getdis(int u,int d,int fa){// printf("ux=%d\n",u);middis[++midcnt]=d;for(int i=fir[u];i;i=nxt[i]){if(vis[v[i]]||v[i]==fa)continue;getdis(v[i],d+1,u);}
}
int look1(int l,int k){int ans=0,r=midcnt;while(l<=r){int mid=(l+r)>>1;if(middis[mid]<k)l=mid+1;elseans=mid,r=mid-1;}return ans;
}
int look2(int l,int k){int ans=0,r=midcnt;while(l<=r){int mid=(l+r)>>1;if(middis[mid]<=k)l=mid+1,ans=mid;elser=mid-1;}return ans;
}
int solve(void){sort(middis+1,middis+midcnt+1);// for(int i=1;i<=midcnt;i++)//     printf("%d ",middis[i]);// getchar();// printf("\n");int mid=0;int l=1;while(l<midcnt&&middis[l]+middis[midcnt]<k)++l;while(l<midcnt&&k-middis[l]>=middis[l]){int l2=look2(l+1,k-middis[l]),l1=look1(l+1,k-middis[l]);if(l2>=l1)mid+=l2-l1+1;l++;}return mid;
}
void divide(int u){// printf("u=%d\n",u);// getchar();vis[u]=true;midcnt=0;getdis(u,0,0);// printf("ok\n");ans+=solve();// printf("an=%d\n",ans);for(int i=fir[u];i;i=nxt[i]){if(vis[v[i]])continue;midcnt=0;getdis(v[i],1,0);ans-=solve();// printf("s=%d\n",ans);root=0;Siz=sz[v[i]];getroot(v[i],u);divide(root);}
}
int main(){scanf("%d %d",&n,&k);for(int i=1;i<=n-1;i++){int a,b;scanf("%d %d",&a,&b);addedge(a,b);addedge(b,a);}Siz=n;f[0]=0x3f3f3f3f;getroot(1,0);divide(root);printf("%lld",ans);return 0;
}