#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
long long pwr(long long a, long long b, long long mod) {
  if (b == 0) return 1;
  long long temp = pwr(a, b / 2, mod);
  temp = (temp * temp) % mod;
  if (b & 1) temp = (temp * a) % mod;
  return temp;
}
long long pwr(long long a, long long b) {
  if (b == 0) return 1;
  long long temp = pwr(a, b / 2);
  temp = (temp * temp);
  if (b & 1) temp = (temp * a);
  return temp;
}
long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }
long long lcm(long long a, long long b) { return (a / gcd(a, b)) * b; }
long long modularInverse(long long a, long long m) { return pwr(a, m - 2, m); }
bool *isPrime;
void generatePrimeSieve(const int lim) {
  isPrime = (bool *)malloc(lim + 1);
  memset(isPrime, true, lim + 1);
  isPrime[0] = false;
  isPrime[1] = false;
  for (int i = 2; i <= lim; ++i)
    if (isPrime[i])
      for (int j = i + i; j <= lim; j += i) isPrime[j] = false;
}
vector<vector<int> > identityMatrix;
vector<vector<int> > mul(const vector<vector<int> > &a,
                         const vector<vector<int> > &b) {
  int n = a.size();
  vector<vector<int> > ans(n, vector<int>(n));
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      for (int k = 0; k < n; ++k) {
        ans[i][j] += ((long long)a[i][k] * b[k][j]) % mod;
        ans[i][j] %= mod;
      }
    }
  }
  return ans;
}
vector<vector<int> > pwr(const vector<vector<int> > &a, long long n) {
  if (n == 0) {
    assert(false);
    return identityMatrix;
  }
  if (n == 1) return a;
  vector<vector<int> > tmp = pwr(a, n / 2);
  tmp = mul(tmp, tmp);
  if (n & 1) tmp = mul(a, tmp);
  return tmp;
}
string str[435];
bool isRemoved[3123];
int main() {
  std::ios::sync_with_stdio(false);
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < n; ++i) cin >> str[i];
  int ans = 0;
  while (true) {
    bool encountered = false;
    for (int i = 0; i < n - 1; ++i) {
      if (str[i] > str[i + 1]) {
        encountered = true;
        for (int j = 0; j < m; ++j) {
          if (str[i][j] > str[i + 1][j]) {
            for (int k = 0; k < n; ++k) str[k][j] = '*';
            break;
          }
        }
        break;
      }
    }
    if (!encountered) break;
  }
  int removed = count(str[0].begin(), str[0].end(), '*');
  cout << removed;
}
