#include <map>

#include <set>

#include <list>

#include <cmath>

#include <ctime>

#include <deque>

#include <stack>

#include <queue>

#include <cctype>

#include <cstdio>

#include <string>

#include <vector>

#include <climits>

#include <cstdlib>

#include <cstring>

#include <iostream>

#include <algorithm>

#define LL long long

#define PI 3.1415926535897932626

using namespace std;

int gcd(int a, int b) {return a % b ==  ? b : gcd(b, a % b);}

#define MAXN 105

const int INF = 0x3f3f3f3f;

int T[],N,K;

int w[MAXN][MAXN],d[MAXN];

int src[MAXN];

typedef pair<int,int> pii;

priority_queue<pii,vector<pii>,greater<pii> > q;

void read()

{

    memset(w,0x3f,sizeof(w));

    for (int i = ; i < N; i++) scanf("%d",&T[i]);

    for (int i = ; i < N; i++)

    {

        int cas = ;

        char ch;

        do

        {

            scanf("%d",&src[cas++]);

            ch = getchar();

        }while (ch != '\n');

        for (int j = ; j < cas; j++)

            for (int k = j; k < cas; k++)

              {

                  w[src[j]][src[k]] = min(w[src[j]][src[k]],abs(src[k] - src[j]) * T[i]);

                  w[src[k]][src[j]] = min(w[src[k]][src[j]],abs(src[k] - src[j]) * T[i]);

              }

    }

}

void SPFA()

{

    bool inq[MAXN];

    memset(inq,false,sizeof(inq));

    memset(d,0x3f,sizeof(d));

    d[] = ;

    queue<int>que;

    while (!que.empty()) que.pop();

    que.push();

    while (!que.empty())

    {

        int u = que.front();que.pop();

        inq[u] = false;

        for (int i = ; i < MAXN; i++)

        {

            if (u == )

            {

                if (d[i] > d[u] + w[u][i])

                {

                    d[i] = d[u] + w[u][i];

                    if (!inq[i])

                    {

                        inq[i] = true;

                        que.push(i);

                    }

                }

            }

            else

            {

                if (d[i] > d[u] + w[u][i] + )

                {

                    d[i] = d[u] + w[u][i] + ;

                    if (!inq[i])

                    {

                        inq[i] = true;

                        que.push(i);

                    }

                }

            }

        }

    }

}

void dijkstra()

{

    memset(d,0x3f,sizeof(d));

    d[] = ;

    q.push(make_pair(d[],));

    while (!q.empty())

    {

        pii t = q.top();q.pop();

        int u = t.second;

        if (t.first != d[u]) continue;

        for (int v = ; v < MAXN; v++)

        {

            if (u == )

            {

                if (d[v] > d[u] + w[u][v])

                {

                    d[v] = d[u] + w[u][v];

                    q.push(make_pair(d[v],v));

                }

            }

            else

            {

                if (d[v] > d[u] + w[u][v] + )

                {

                    d[v] = d[u] + w[u][v] + ;

                    q.push(make_pair(d[v],v));

                }

            }

        }

    }

}

int main()

{

    //freopen("sample.txt","r",stdin);

    while (scanf("%d%d",&N,&K) != EOF)

    {

        read();

        SPFA();

        //dijkstra();

        if (d[K] == INF) puts("IMPOSSIBLE");

        else printf("%d\n",d[K]);

    }

    return ;

}