// Problem: P6681 [CCO2019] Bad Codes
// Contest: Luogu
// URL: https://www.luogu.com.cn/problem/P6681
// Memory Limit: 512 MB
// Time Limit: 1000 ms
// 
// Powered by CP Editor (https://cpeditor.org)

#include<bits/stdc++.h>
using namespace std;
const int N=1005;
const int M=5e6+5;
const int inf=1e9+7;

bool med;

int n,m,a[N],s,t;
string S[N];

int id(int i,int j){return (i-1)*(m+1)+j+1;}

int cnt,head[M];
struct edge{
	int to,w,nxt;
}e[M<<1];
void add(int u,int v,int w){
	e[++cnt]={v,w,head[u]},head[u]=cnt;
}

struct node{
	int dis,id;
	bool operator < (const node &a) const{
		return dis>a.dis;
	}
};
int f[M];
bool vis[M];
priority_queue<node> q;

void dij(){
	for(int i=1;i<=t;++i) f[i]=inf;
	f[s]=0,q.push({0,s});
	while(!q.empty()){
		int u=q.top().id; q.pop();
		if(vis[u]) continue;
		vis[u]=1;
		for(int i=head[u];i;i=e[i].nxt){
			int v=e[i].to;
			if(f[v]>f[u]+e[i].w){
				f[v]=f[u]+e[i].w;
				if(!vis[v]) q.push({f[v],v});
			}
		}
	}
}

bool mde;

signed main(){
//	cerr << (&med-&mde)/1048576.0 << "MB\n";
	
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;++i) cin >> S[i],a[i]=S[i].size();
	
	for(int i=1;i<=n;++i){
		for(int j=0;j<a[i];++j){
			int dl=a[i]-j;
			for(int k=1;k<=n;++k){
				if(!j && k==i) continue;
				if(a[k]>=dl){
					if(S[i].substr(j,dl)==S[k].substr(0,dl)){
						add(id(i,dl),id(k,a[k]-dl),a[k]-dl);
					}
				}else{
					if(S[i].substr(j,a[k])==S[k].substr(0,a[k])){
						add(id(i,dl),id(i,dl-a[k]),0);
					}
				}
			}
		}
	}
	s=id(n,m)+1,t=s+1;
	for(int i=1;i<=n;++i) add(s,id(i,a[i]),a[i]),add(id(i,0),t,0);
	dij();
	return printf("%d",f[t]==inf ? -1 : f[t]),0;
}