count how many distinct subsequence of length j of a string

/* count how many distinct subsequence of length j of a string upto position i */



string s;int n;
ll dp[105][105];

void giveres(){
    int last[300];
    mem(last,-1);
    for(int i = 1;i<=n;i++){

        for(int j = 1;j<=i;j++){
            dp[i][j] = dp[i-1][j] + dp[i-1][j-1] ;
            if(last[s[i-1]]!=-1)dp[i][j] = dp[i][j] - dp[last[s[i-1]]][j-1];
        }
        last[s[i-1]] = i-1;
    }
}

void print(){
    for(int i = 0;i<=n;i++){
        for(int j = 0;j<=i;j++){
            cout<<i<<" "<<j<<" "<<dp[i][j]<<endl;
        }
    }
}


int main()
{
    booster()
    ///read("input.txt");

    ll k;
    cin>>n>>k;
    cin>>s;
    ll ans = 0;
    for(int i = 0;i<=n;i++)
    dp[i][0] = 1;
    giveres();
    //print();
    for(int i = n;i>=0;i--){
      // cout<<i<<" "<<dp[n][i]<<endl;
        ans+=(min(dp[n][i],k))*(n-i);
        k-=min(dp[n][i],k);
        if(!k)break;
    }
    if(!k)cout<<ans;
    else cout<<-1;




    return 0;
}