#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

void solve(){

int N, M; cin >> N >> M;

string str[N];

for (int i = 0; i < N; i++) cin >> str[i];

bool found = false;

bool removed[M]; memset(removed, 0, sizeof removed);

for (int i = 0; i < M; i++) {

for (int j = 0; j < N-1; j++) {

if (str[j][i] > str[j+1][i]) {

bool skip = false;

for (int k = 0; k < i; k++) {

if (removed[k]) continue;

skip |= str[j][k] < str[j+1][k];

}

if (skip) continue;

removed[i] = true;

}

}

}

int result = 0;

for (int i = 0; i < M; i++) result += removed[i];

cout << result;

}

int main()

{

solve();

return 0;

}