#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;
using namespace __gnu_pbds;

template<typename T>
ostream_iterator<T> oit(const string &s = " "){ return ostream_iterator<T>(cout,s.c_str()); }
inline auto rep(int l, int r) { return views::iota(min(l, r), r); }
inline auto rep(int n) { return rep(0, n); }
inline auto rep1(int l, int r) { return rep(l, r + 1); }
inline auto rep1(int n) { return rep(1, n + 1); }
inline auto per(int l, int r) { return rep(l, r) | views::reverse; }
inline auto per(int n) { return per(0, n); }
inline auto per1(int l, int r) { return per(l, r + 1); }
inline auto per1(int n) { return per(1, n + 1); }
#define A(a) begin(a),end(a)
inline auto len = ranges::ssize;

struct chash {
    static uint64_t splitmix64(uint64_t x) {
        // http://xorshift.di.unimi.it/splitmix64.c
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};
template<typename T, typename U> using pb_map = gp_hash_table<T, U, chash>;
template<typename T> using pb_set = gp_hash_table<T, null_type, chash>;
#define K first
#define V second

using ll = long long;
using ld = long double;

using vi = vector<int>;
using vii = vector<vector<int>>;
typedef vector<ll> vll;
using pll = pair<ll,ll>;
using pii = pair<int,int>;

constexpr ll NN = 2e5, M = 1000000007, L = 20;

ll f1[NN], f2[NN];
ll inv(ll a, ll b=M) { return 1 < a ? b - inv(b % a, a) * b / a : 1; } // inv a mod b
ll choose(ll n, ll k) { return f1[n] * f2[k] % M * f2[n - k] % M; } // n choose k

struct T{
    int dist,len;
    char c;
};
bool operator<(T l,T r){
    return l.dist > r.dist;
}

void run()
{
    int n,k; cin >> n >> k;

    map<string,int> mp;
    for(string s; int _ : rep(n)){
        int c;
        cin >> s >> c;
        mp[s]=c;
    }

    priority_queue<T> pq;
    vector<array<bool,26>> done(k+10); //len,letter
    for(auto &v : done) ranges::fill(v,0);

    if(k%2){
        for(char c : rep1('a','z')) pq.push(T{0,1,c});
    }else{
        for(auto [s,cost] : mp){
            if(s[0]==s[1]) pq.emplace(T{cost,2,s[0]});
        }
    }

    while(not pq.empty()){
        auto node = pq.top(); pq.pop();
        if(done[node.len][node.c-'a']) continue;
        if(node.len == k)
            return void(cout << node.dist << '\n');
        
        done[node.len][node.c-'a']=1;

        for(char nc : rep1('a','z')){
            string need1 {node.c,nc},need2={nc,node.c};
            if(done[node.len+2][nc-'a']) continue; //optmizatoin
            if(mp.contains(need1) and mp.contains(need2)){
                int weight = mp[need1]+mp[need2];
                pq.push({node.dist+weight,node.len+2,nc});
            }
        }
    }

    //NICK CHECK IF THE PQ WORKS RIGHT
    cout << "-1\n";
}

int main()
{
    //KING OF THE WORLD...... U.W.T.B
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    run();
}