#include <bits/stdc++.h>

using namespace std;
int n,m;
char field[3001][3001];
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    freopen("ROWCUT.inp","r",stdin);
    freopen("ROWCUT_trau.out","w",stdout);
    cin >> n >> m;
    for (int i=1;i<=n;i++)
        for (int j=1;j<=m;j++)
            cin >> field[i][j];
    int res = 0;
    for (int i=1;i<=n;i++)
    {
        bool ok = false;
        string tmp;
        for (int j=1;j<=n-i;j++)
            tmp+=field[j][1];
        for (int j=2;j<=m;j++)
        {
            string o_tmp;
            for (int k=1;k<=n-i;k++)
                o_tmp += field[k][j];
            if (o_tmp == tmp)
            {
                ok = true;
                break;
            }
        }
        if (ok) break;
        res = i;
    }
    if (!res) cout << -1;
    else cout << res;
}