#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007;
const long long INF = 1000000007;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
string gh = "here";
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, m;
  int ans = 0;
  bool on = true;
  cin >> n >> m;
  vector<string> col(m);
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    for (int j = 0; j < m; j++) {
      col[j].push_back(s[j]);
    }
  }
  while (on) {
    on = false;
    for (int i = 0; i < n - 1; i++) {
      for (int j = 0; j < m; j++) {
        if (col[j][i] > col[j][i + 1]) {
          col.erase(col.begin() + j);
          m--;
          j--;
          on = true;
          ans++;
        } else if (col[j][i] == col[j][i + 1]) {
          continue;
        } else {
          break;
        }
      }
    }
  }
  cout << ans;
  return 0;
}
