#include "bits/stdc++.h"
#define rep(i,a,n) for(auto i=a;i<=n;i++)
#define per(i,a,n) for(auto i=n;i>=a;i--)
#define pb push_back
#define mp make_pair
#define FI first
#define SE second
#define all(A) A.begin(),A.end()
#define sz(A) (int)A.size()
template<class T> inline bool chmax(T &a, T b) {if(a<b) {a=b; return 1;} return 0;}
template<class T> inline bool chmin(T &a, T b) {if(b<a) {a=b; return 1;} return 0;}
using namespace std;

string to_string(const string& s) {return '"' + s + '"';}
string to_string(const char* s) {return to_string((string) s);}
template<typename A, typename B> string to_string(pair<A, B> p) {return "(" + to_string(p.FI) + ", " + to_string(p.SE) + ")";}
template<typename A> string to_string(A v) 
{
    bool first = 1;
    string res = "{";
    for(const auto &x: v) 
    {
        if (!first) res += ", ";
        first = 0;
        res += to_string(x);
    }
    res += "}";
    return res;
}

void debug_out() {cerr << endl;} 
template<typename Head, typename... Tail> void debug_out(Head H, Tail... T) 
{
    cerr << " " << to_string(H);
    debug_out(T...);
}
#ifndef ONLINE_JUDGE
    #define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
    #define debug(...) if(0) puts("No effect.")
#endif

using ll = long long;
//using LL = __int128;
using pii = pair<int,int>;
using vi = vector<int>;
using db = double;
using ldb = long double;

const int maxn = 1000;
const int inf = 0x3f3f3f3f;
// const int mod = 1e9+7;

int main()
{
    ios::sync_with_stdio(0); cin.tie(0);

    int n,k; cin >> n >> k;
    map<string,int> w;
    rep(i,1,n)
    {
        string s; int c; cin >> s >> c;
        if(w.count(s)==0) w[s] = c;
        else chmin(w[s], c);
    }
    static int dp[2][26];
    int cur = 0;
    if(k&1) rep(i,0,25) dp[cur][i] = 0;
    else 
    {
        rep(i,0,25)
        {
            string s = "aa";
            s[0] += i; s[1] += i;
            dp[cur][i] = w.count(s) ? w[s] : inf;
        }
    }
    k = (k+1)/2;
    rep(_,2,k)
    {
        int old = cur; cur ^= 1;
        memset(dp[cur],63,sizeof dp[cur]);
        rep(i,0,25) if(dp[old][i]!=inf) rep(j,0,25)
        {
            string s = "aa";
            string t = "aa";
            s[0] += i; s[1] += j;
            t[0] += j; t[1] += i;
            if(w.count(s) && w.count(t)) chmin(dp[cur][j], dp[old][i]+w[s]+w[t]);
        }
    }
    int ans = inf;
    rep(i,0,25) chmin(ans, dp[cur][i]);
    if(ans==inf) puts("-1");
    else printf("%d\n",ans);
    return 0;
}