//2022.1.28 by ljz
//email [email protected]
//if you find any bug in my code
//please tell me
#include<bits/stdc++.h>
//#include<ext/pb_ds/tree_policy.hpp>
//#include<ext/pb_ds/assoc_container.hpp>
using namespace std;
//using namespace __gnu_pbds;
//using namespace __gnu_cxx;
#define res int
#define LL long long
#define Inf 0x3f3f3f3f
#define sup 0x7fffffff
#define inf 0x3f3f3f3f
#define INF 2000000000000000000
//#define unl __int128
#define eps 1e-10
#define RG
#define db double
#define pc(x) __builtin_popcount(x)
#define ctz(x) __builtin_ctz(x)
//#define pc(x) __builtin_popcountll(x)
typedef pair<int,int> Pair;
//#define poly vector<int>
#define mp make_pair
#define fi first
#define se second
#define pi acos(-1.0)
#define pb push_back
#define ull unsigned LL
#define uint unsigned int
#define lowbit(x) ((x)&-(x))
#define gc getchar
#define ld long db
//template <class T>using Tree=tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
//inline int gc() {
//    static char buf[100000],*p1,*p2;
//    return p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;
//}
//char sr[1<<21],z[20];
//int C=-1,Z=0;
//inline void Ot(){fwrite(sr,1,C+1,stdout),C=-1;}
//inline void print(RG LL x){
//    if(C>1<<20)Ot();if(x<0)sr[++C]='-',x=-x;
//    while(z[++Z]=x%10+48,x/=10);
//    while(sr[++C]=z[Z],--Z);
//}
template <typename T> inline void Read(T &x) {
    res c=gc();
    bool f=false;
    for(x=0;!isdigit(c);c=gc())if(c=='-')f=true;
    for(;isdigit(c);c=gc())x=x*10+c-'0';
    if(f)x=-x;
}
inline int read() {
    res s=0,ch=gc(),w=1;
    while(ch<'0'||ch>'9'){
        if(ch=='-')w=-1;
        else if(ch==EOF)break;
        ch=gc();
    }
    while(ch>='0'&&ch<='9')s=s*10+ch-'0',ch=gc();
    return s*w;
}
inline LL Read() {
    RG LL s=0;
    res ch=gc(),w=1;
    while(ch<'0'||ch>'9'){
        if(ch=='-')w=-1;
        else if(ch==EOF)break;
        ch=gc();
    }
    while(ch>='0'&&ch<='9')s=s*10+ch-'0',ch=gc();
    return s*w;
}
inline void write(RG __int128 x){
    if(x>10)write(x/10);
    putchar(int(x%10)+'0');
}
const int kcz=998244353;
const int G=3,GI=332748118;
//inline void add(res &x,const res &y){
//    x+=y,x>=kcz?x-=kcz:1;
//}
//inline int Add(const res &x,const res &y){
//    return x+y>=kcz?x+y-kcz:x+y;
//}
//inline int mul(const res &x,const res &y){
//    return int(1ll*x*y%kcz);
//}
#define add(x,y) ((x)+=(y),(x)>=kcz?(x)-=kcz:1)
#define Add(x,y) ((x)+(y)>=kcz?(x)+(y)-kcz:(x)+(y))
#define mul(x,y) (int)((LL)(x)*(y)%kcz)
#define Mul(x,y,d) (int)((ull)(x)*(y)/(d)%kcz)
inline int qpow(res x,res y=kcz-2){
    res ret=1;
    while(y){
        if(y&1)ret=mul(ret,x);
        x=mul(x,x),y>>=1;
    }
    return ret;
}
inline int qpow(res x,res y,const res &ljc){
    res ret=1;
    while(y){
        if(y&1)ret=(int)(1ll*ret*x%ljc);
        x=(int)(1ll*x*x%ljc),y>>=1;
    }
    return ret;
}
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
//cloclim_t start=cloclim();
//inline void clim(){
//    if(1.0*(cloclim()-start)/CLOCKS_PER_SEC>0.1)exit(0);
//}
//2022.1.28 by ljz
//email [email protected]
//if you find any bug in my code
//please tell me
namespace MAIN{
    int n,k;
    int a[26][26];
    LL f[101][26];
    char str[10];
#define ckmin(x,y) ((x)>(y)?(x)=(y):1)
    inline void MAIN(){
        n=read(),k=read();
        memset(f,0x3f,sizeof(f));
        memset(a,0x3f,sizeof(a));
        for(res i=1;i<=n;i++){
            scanf("%s",str+1);
            res c=read();
            ckmin(a[str[1]-'a'][str[2]-'a'],c);
        }
        for(res i=0;i<26;i++)f[1][i]=0;
        for(res i=2;i<=(k+1)/2;i++){
            for(res j=0;j<26;j++){
                for(res l=0;l<26;l++){
                    ckmin(f[i][j],f[i-1][l]+a[l][j]+a[j][l]);
                }
            }
        }
        if(~k&1){
            LL ans=inf;
            for(res i=0;i<26;i++)ans=min(ans,f[k/2][i]+a[i][i]);
            printf("%lld\n",ans>=10000?-1:ans);
        }
        else{
            LL ans=inf;
            for(res i=0;i<26;i++)ans=min(ans,f[k/2+1][i]);
            printf("%lld\n",ans>=10000?-1:ans);
        }
    }
}
int main(){
//    srand(time(0));
//    freopen("1.in","r",stdin);
//    freopen("1.out","w",stdout);
    res Case=1;
    for(res T=1;T<=Case;T++)MAIN::MAIN();
//    Ot();
    return 0;
}