#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
inline int read(){
    char ch=getchar();
    while(!isdigit(ch) && ch!='-') ch=getchar();
    int x=0,ff=1; if(ch=='-') ff=-1,ch=getchar();
    while(isdigit(ch)) x=(x<<3) + (x<<1) + (ch^48),ch=getchar();
    return x*ff;
}
const int N=4e5+5;
int n,m,a[N],b[N],ind[N],ek[N],ans[N],ew[N];
vector<int> vt[N],e[N],qe,ve; char ch[N]; queue<int> q;
void ecal(int i,int w){
    ew[i]=w; qe.push_back(i);
    for(int x:vt[i]) {
        (a[x]+=ew[i])%=3;
        if((--ind[x])==1) q.push(x);
    }
}
int qtop(){ int s=0; ve.clear();
    while(!q.empty()){
        int u=q.front(),w=(3-a[u])%3; ve.push_back(u); q.pop();
        for(int i:e[u]) if(!~ew[i]) ecal(i,w);
        if(a[u]) return -1;
    }
    for(int x:qe) s+=ew[x];
    return s;
}
int main(){
    n=read(); m=read(); scanf("%s",ch+1);
    for(int i=1;i<=n;i++) a[i]=(ch[i]=='R'?0:(ch[i]=='G'?1:2)),b[i]=a[i];
    for(int i=1;i<=m;i++){
        ek[i]=read(); vt[i].resize(ek[i]); ew[i]=-1;
        for(int j=0,x;j<ek[i];j++) vt[i][j]=x=read(),e[x].push_back(i),ind[x]++;
    }
    for(int i=1;i<=n;i++) if(ind[i]==1) q.push(i);
    int ww=qtop(),ans=0;
    if(~ww) ans+=ww;
    else {puts("impossible"); return 0;}
    for(int i=1;i<=m;i++) if(!~ew[i]) {
        int mn=1e9,d=4;
        for(int j=0;j<3;j++){
            qe.clear(),ecal(i,j); int ww=qtop();
            if(~ww) {
                if(ww<mn) mn=ww,d=j;
            }
            for(int x:qe) ew[x]=-1;
            for(int x:ve) ind[x]=2,a[x]=b[x];
        }
        if(d==4) {puts("impossible"); return 0;}
        else {
            qe.clear(),ecal(i,d),ans+=qtop();
        }
    }
    for(int i=1;i<=n;i++) if(a[i]) {puts("impossible"); return 0;}
    printf("%d\n",ans);
    return 0;
}