codeforces813C The Tag Game

C. The Tag Game

time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

Alice got tired of playing the tag game by the usual rules so she offered Bob a little modification to it. Now the game should be played on an undirected rooted tree of n vertices. Vertex 1 is the root of the tree.

Alice starts at vertex 1 and Bob starts at vertex x (x ≠ 1). The moves are made in turns, Bob goes first. In one move one can either stay at the current vertex or travel to the neighbouring one.

The game ends when Alice goes to the same vertex where Bob is standing. Alice wants to minimize the total number of moves and Bob wants to maximize it.

You should write a program which will determine how many moves will the game last.

Input

The first line contains two integer numbers n and x (2 ≤ n ≤ 2·105, 2 ≤ x ≤ n).

Each of the next n - 1 lines contains two integer numbers a and b (1 ≤ a, b ≤ n) — edges of the tree. It is guaranteed that the edges form a valid tree.

Output

Print the total number of moves Alice and Bob will make.

Examples

input

4 31 22 32 4

output

4

input

5 21 22 33 42 5

output

6

题解：

找到bob比alice先到的点中深度最大的一个 答案为(该点深度-1)*2

代码：

#include <iostream>#include <cstdio>#include <cstring>#include <vector>using namespace std;const int maxn=200000+10;vector<int>A[maxn];int a[maxn];int b[maxn];int n,p;int ans=0;inline void dfs(int x,int f,int d,int *a){a[x]=d;for(int i=0;i<A[x].size();i++){int u=A[x][i];if(u==f)continue;dfs(u,x,d+1,a);}}int main(){//freopen("a.in","r",stdin);//freopen("a.out","w",stdout);scanf("%d %d",&n,&p);int x,y;for(int i=1;i<n;i++){scanf("%d %d",&x,&y);A[x].push_back(y);A[y].push_back(x);}dfs(1,0,1,a);dfs(p,0,1,b);for(int i=1;i<=n;i++){if(a[i]>b[i])ans=max(ans,(a[i]-1)*2);}printf("%d\n",ans);return 0;}