dfs(int u)
{
    if(找到了||走不下去了)
    {
        return；
 
    }
    开始下一个情况（dfs(u+1)）;
 
}

#include <iostream>
#include <bits/stdc++.h>
 
using namespace std;
 
typedef long long ll;
 
const int N = 20;
 
int n;
char g[N][N];
int l[N],ug[N],ng[N];
 
void dfs(int u)
{
    if(u == n)
    {
        for(int i = 0; i < n; i ++)
        {
            cout << g[i] << endl;
        }
        cout << endl;
        return ;
    }
    for(int i = 0; i < n; i ++)
    {
        if(!l[i] && !ug[i + u] && !ng[i - u + n])
        {
            g[u][i] = 'Q';
            l[i] = ug[i + u] = ng[i - u + n] = 1;
            dfs(u + 1);
            l[i] = ug[i + u] = ng[i - u + n] = 0;
            g[u][i] = '.';
        }
    }
 
}
int main()
{
    cin >> n;
    
    for(int i = 0; i < n; i ++)
    {
        for(int j = 0; j < n; j ++) 
        {
            g[i][j] = '.';
        }
    }
    
    dfs(0);
    
    return 0;
}
 

queue<int> q;
st[0] = true; // 表示1号点已经被遍历过
q.push(0);
 
while (q.size())
{
    int t = q.front();
    q.pop();
 
    for (int i = h[t]; i != -1; i = ne[i])
    {
        int j = e[i];
        if (!s[j])
        {
            st[j] = true; // 表示点j已经被遍历过
            q.push(j);
        }
    }
}

#include <iostream>
#include <bits/stdc++.h>
#define x first
#define y second
 
using namespace std;
 
const int N = 1000 + 10;
 
int n;
typedef pair<int,int> PII;
PII q[N * N];
bool st[N][N];
char g[N][N];
 
int dx[4] = {-1, 0, 1, 0},dy[4] = {0, 1, 0, -1};
 
void bfs(int sx, int sy, int &tl, int &bd)
{
 
    int tt = 0,hh = 0;
 
    st[sx][sy] = true;
    q[0] = {sx, sy};
 
    while(hh <= tt)
    {
        PII t = q[hh ++];
        tl ++;
        bool is_bd = false;
 
        for(int i = 0; i < 4; i ++)
        {
            int x = t.x + dx[i],y = t.y + dy[i];
            if(x < 0 || x >= n || y < 0 || y >= n || st[x][y]) continue;
            if(g[x][y] == '.')
            {
                is_bd = true;
                continue;
            }
 
            st[x][y] = true;
            q[++ tt] = {x, y};
        }
 
        if(is_bd) bd++;
    }
 
}
 
int main(){
 
   cin >> n;
   
   for(int i = 0;i < n;i++) cin >> g[i];
 
   int cnt  = 0;
 
   for(int i = 0;i < n;i++)
    for(int j = 0;j < n;j++)
    {
        if(!st[i][j] && g[i][j] == '#')
        {
            int tl = 0,bd = 0;
 
            bfs(i, j, tl, bd);
 
            if(tl == bd) cnt ++;
        }
    }
 
    cout << cnt << endl;
    return 0;
}

int prim()
{
    memset(dist, 0x3f, sizeof dist);
    int res = 0;
    dist[1] = 0;
    for(int i = 0; i < n; i ++)
    {
        int t = -1;
        for(int j = 1; j <= n; j ++)
        if(!st[j] && (t == -1 || dist[j] < dist[t]))
           t = j;
           st[t] = true;
           res += dist[t];
        for(int j = 1; j <= n; j ++)
            if(dist[j] > g[t][j] && !st[i])
                {
                    dist[j] = g[t][j];
                    pre[j] = t;
                }
    }
    return res;
}

9 16
0 1 8
0 2 12
1 4 9
1 3 25
1 2 13
2 6 21
2 3 14
2 6 12
3 5 8
3 7 12
3 8 16
4 5 19
4 3 20
5 7 11
7 8 6
6 8 11

#include<bits/stdc++.h>
 
using namespace std;
 
const int N = 2e5+5;
int n,m;
 
struct Edge
{
	int u, v, w;
	bool operator<(const Edge &a) const
	{
		return w<a.w;
	}
}edge[N];
 
int p[N];
int find(int x)
{
	return p[x] == x ? x : p[x] = find(p[x]);
}
 
 
int main()
{
	int n, m;
	scanf("%d%d",&n, &m);
	for(int i=0;i<m;i++)
    {
		int u, v, w;
		scanf("%d%d%d",&u, &v, &w);
		edge[i] = {u,v,w};
	}
	sort(edge, edge + m);
	
	for(int i = 1; i <= n; i ++) p[i] = i;
	
	int cnt = 0, sum = 0;
	
	for(int i = 0; i < m; i ++)
    {
		int a = edge[i].u, b=edge[i].v, w = edge[i].w;
		a = find(a);
		b = find(b);
		if(a != b)
		{
			cnt ++;
			sum += w;
			p[a] = b;
		}
	}
	if(cnt < n - 1) puts("impossible");
	else printf("%d", sum);
}