#include <bits/stdc++.h>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
#define in(x) freopen(x, "r", stdin)
#define out(x) freopen(x, "w", stdout)
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("-O3")
#define F first
#define S second
#define pb push_back
#define N +400500
#define M ll(1e9 + 7)
#define sz(x) (int)x.size()
#define re return
#define oo ll(1e18)
#define el '\n'
using namespace std;
//using namespace __gnu_pbds;
//typedef tree <int, null_type, less_equal <int> , rb_tree_tag, tree_order_statistics_node_update> ordered_set;
typedef long long ll;
typedef long double ld;
bool go[51][1501][1501];
ll f[51][1501], ans, hsh[51][1501], p[51];
int i, j, k, m, p1, n;
string s[51][1501];
ll sm(ll x, ll y) {return (x + y) % M;}
int main()
{
srand(time(0));
ios_base::sync_with_stdio(0);
iostream::sync_with_stdio(0);
ios::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);
// in("input.txt");
// out("output.txt");
cin >> n >> m;
p[0] = 1;
for (i = 1; i <= n; i++) p[i] = (p[i - 1] * 31) % M;
for (i = 1; i <= n; i++)
for (j = 1; j <= m; j++)
{
cin >> s[i][j];
hsh[i][j] = 0;
for (k = 0; k < i; k++)
hsh[i][j] = sm(hsh[i][j], (s[i][j][k] - 'a' + 1) * p[k] % M);
}
for (i = 1; i < n; i++)
{
for (j = 1; j <= m; j++)
{
for (k = 1; k <= m; k++)
{
if (hsh[i][j] * 31 % M == (hsh[i + 1][k] - (s[i + 1][k][0] - 'a' + 1) + M) % M) go[i][j][k] = 1;
if (hsh[i][j] == (M + hsh[i + 1][k] - (s[i + 1][k][i] - 'a' + 1) * p[i] % M) % M) go[i][j][k] = 1;
}
}
}
for (i = 1; i <= m; i++) f[1][i] = 1;
for (i = 1; i < n; i++)
for (j = 1; j <= m; j++)
for (k = 1; k <= m; k++)
if (go[i][j][k]) f[i + 1][k] = sm(f[i][j], f[i + 1][k]);
ans = 0;
for (i = 1; i <= m; i++) ans = sm(f[n][i], ans);
cout << ans;
}
