#include<cstdio>
#include<algorithm>
#include<queue>
#include<cstring>
using namespace std;
char s1[10],s2[10];
int pp,len1,len2,d[17000000],pw[10];
bool vis[17000000],cant[8];
int st[17000000],En;
struct Node{int u,d;Node(const int &u,const int &d){this->u=u;this->d=d;}Node(){}
};
bool operator < (const Node &a,const Node &b){return a.d>b.d;
}
priority_queue<Node>Heap;
int main(){
//  freopen("j.in","r",stdin);memset(d,0x7f,sizeof(d));pw[0]=1;for(int i=1;i<=8;++i){pw[i]=pw[i-1]*8;}while(scanf("%s%d%s",s1,&pp,s2)!=EOF){memset(cant,0,sizeof(cant));bool flag=1;while(!Heap.empty()){Heap.pop();}En=0;int U=0,len1=strlen(s1),len2=strlen(s2);for(int i=0;i<len2;++i){if(cant[s2[i]-'a'+1]){flag=0;break;}cant[s2[i]-'a'+1]=1;}if(!flag){puts("-1");continue;}for(int i=0;i<len1;++i){U=U*8+s1[i]-'a'+1;}U=U*8+pp;int goal=0;for(int i=0;i<len2;++i){goal=goal*8+s2[i]-'a'+1;}Heap.push(Node(U,0));d[U]=0;st[++En]=U;while(!Heap.empty()){Node now=Heap.top(); Heap.pop();if(!vis[now.u]){if(now.u/8==goal){break;}vis[now.u]=1;int U=now.u;int len=0;int gbp=U%8; U/=8;memset(cant,0,sizeof(cant));
//              char S[10];while(U){cant[U%8]=1;
//                  S[len++]=U%8+'a'-1;++len;U/=8;}
//              for(int i=0;i<len;++i){
//                  putchar(S[i]);
//              }
//              puts("");//plusif(len<7){U=now.u;U-=(U%pw[len-gbp+1]);U*=8;U+=(now.u%pw[len-gbp+1]);for(int i=1;i<=7;++i){if(!cant[i]){int tU=U+i*pw[len-gbp+1];++tU;if(d[tU]>d[now.u]+2){d[tU]=d[now.u]+2;Heap.push(Node(tU,d[tU]));st[++En]=tU;}}}}//deleteif(len>0){if(gbp>0){U=now.u%pw[len-gbp+1];int tmp=now.u;tmp/=pw[len-gbp+2];tmp*=pw[len-gbp+1];U+=tmp;--U;if(d[U]>d[now.u]+2){d[U]=d[now.u]+2;Heap.push(Node(U,d[U]));st[++En]=U;}}if(gbp<len){U=now.u%pw[len-gbp];int tmp=now.u;tmp/=pw[len-gbp+1];tmp*=pw[len-gbp];U+=tmp;if(d[U]>d[now.u]+2){d[U]=d[now.u]+2;Heap.push(Node(U,d[U]));st[++En]=U;}}}//moveif(gbp>0){if(d[now.u-1]>d[now.u]+1){d[now.u-1]=d[now.u]+1;Heap.push(Node(now.u-1,d[now.u-1]));st[++En]=now.u-1;}}if(gbp<len){if(d[now.u+1]>d[now.u]+1){d[now.u+1]=d[now.u]+1;Heap.push(Node(now.u+1,d[now.u+1]));st[++En]=now.u+1;}}}}printf("%.1f\n",(double)(*min_element(d+goal*8,d+goal*8+8))*0.5);for(int i=1;i<=En;++i){d[st[i]]=2000000000;vis[st[i]]=0;}}return 0;
}