#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001



vector<int> get(string s,string t){
	vector<int> ret(s.size()+1,0);
	int c = 0;
	rep(i,s.size()){
		if(s[i] == t[c]){
			c++;
		}
		ret[i+1] = c;
	}
	return ret;
}

int main(){
	
	vector<string> s;
	int ans = 0;
	int n,m;
	cin>>n>>m;
	s.resize(n);
	rep(i,n)cin>>s[i];
	
	vector<vector<int>> x(n-1);
	rep(i,n-1){
		x[i] = get(s[0],s[i+1]);
	}
	rep(i,n)reverse(s[i].begin(),s[i].end());
	vector<vector<int>> y(n-1);
	rep(i,n-1){
		y[i] = get(s[0],s[i+1]);
	}
	rep(i,n)reverse(s[i].begin(),s[i].end());
	rep(i,m+1){
		rep(j,26){
			if(i!=m && s[0][i]=='a'+j)continue;
			bool f = true;
			rep(k,n-1){
				if(x[k][i] + y[k][m-i] < m-1){
					f = false;
					break;
				}
				if(s[k+1][x[k][i]]!='a'+j){
					f = false;
					break;
				}
			}
			if(f)ans++;
			
		}
	}
	
	cout<<ans<<endl;
	
	return 0;
}