#include <bits/stdc++.h>
using namespace std;
#define int long long
const int mod = 1e9 + 7;
signed main(){
int n, k; cin >> n >> k;
vector<vector <int>> dp(n + 1, vector<int>(k + 1));
vector<vector<string>> inp(n + 1, vector<string>(k + 1));
for(int i = 1;i <= n;i++){
for(int j = 1;j <= k;j++) cin >> inp[i][j];
}
for(int i = 1;i <= k;i++) dp[1][i] = 1;
for(int i = 2;i <= n;i++){
map <string, int> cnt;
for(int j = 1;j <= k;j++){
cnt[inp[i - 1][j]] += dp[i - 1][j];
cnt[inp[i - 1][j]] %= mod;
}
for(int j = 1;j <= k;j++){
string a = inp[i][j].substr(0, i - 1);
string b = inp[i][j].substr(1, i - 1);
if(a != b) dp[i][j] = (cnt[a] + cnt[b]) % mod;
else dp[i][j] = cnt[a];
}
}
int ans = 0;
for(int i = 1;i <= k;i++) (ans += dp[n][i]) %= mod;
cout << ans;
}
