#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define ull unsigned long long
#define pb push_back
#define pll pair<ll, ll>
#define pii pair<int, int>
#define fi first
#define se second
using namespace std;
const int N = 2e3+5;
const int M = 1e5+5;
const int mod = 1e9+7;
const ll base = 327;
const ll inf = 1e15+7;
int c[N], d[N], ans, tong, dp[N], h[N];
int n, m, k;
int a[52][N][52], b[N];
vector<int> adj[N], vi, gr[N];
void add(int& x, int y)
{
x += y;
if(x >= mod)x -= mod;
}
struct BIT
{
int n;
vector<ll> fe;
BIT(){}
void init(int _n)
{
n = _n;
fe.assign(n+1, 0);
}
void add(int id, ll x)
{
for(; id <= n; id += id & -id)fe[id] += x;
}
void add(int l, int r, ll x)
{
add(l, x);
add(r+1, -x);
}
ll get(int id)
{
ll res = 0;
for(; id; id -= id & -id)res += fe[id];
return res;
}
}L, R;
struct query
{
int x, y, l, r;
}q[N];
vector<string> s[N];
int get(int i, int j, int l, int r)
{
return (a[i][j][r]-1ll*a[i][j][l-1]*c[r-l+1]%mod+mod)%mod;
}
void sol()
{
srand((int)(time(0)));
cin >> n >> m;
for(int i = 0; i < 26; i ++)b[i] = rand() % mod;
c[0] = 1;
for(int i = 1; i <= n; i ++)c[i] = 1ll*c[i-1]*base%mod;
for(int i = 1; i <= n; i ++)
{
s[i].resize(m);
for(int j = 0; j < m; j ++)
{
cin >> s[i][j];
for(int p = 1; p <= i; p ++)
a[i][j][p] = (1ll*a[i][j][p-1]*base+b[s[i][j][p-1]-'a'])%mod;
}
}
map<int, int> mp;
for(int j = 0; j < m; j ++)++mp[a[1][j][1]];
for(int i = 2; i <= n; i ++)
{
for(int j = 0; j < m; j ++)
{
dp[j] = 0;
k = a[i][j][i-1];
if(mp.count(k))add(dp[j], mp[k]);
k = get(i, j, 2, i);
if(k == a[i][j][i-1])continue;
if(mp.count(k))add(dp[j], mp[k]);
}
mp.clear();
for(int j = 0; j < m; j ++)
{
k = a[i][j][i];
mp[k] += dp[j];
if(mp[k] >= mod)mp[k] -= mod;
}
//cout << mp.size() << '\n';
}
for(int j = 0; j < m; j ++)add(ans, dp[j]);
cout << ans;
}
int main()
{
cin.tie(0);
cout.tie(0);
ios_base::sync_with_stdio(0);
#define task "test"
if(fopen(task".inp", "r"))
{
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
int test = 1;
//cin >> test;
while(test -- > 0)sol();
return 0;
}
