#include<bits/stdc++.h>
using namespace std;
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#define rep(i,n) for (int i=0;i<(long long)(n);i++)
#define all(v) v.begin(),v.end()
using ll=long long;
using pll=pair<ll,ll>;
using tll=tuple<ll,ll,ll>;
const ll INF=(1ll<<60);
template<class T> void chmin(T &a,T b){
    if(a>b){
        a=b;
    }
}
template<class T> void chmax(T &a,T b){
    if(a<b){
        a=b;
    }
}
int main(){
    ll n,k;
    cin >> n >> k;
    vector<deque<char>> s(n);
    rep(i,n){
        rep(j,k){
            char c;
            cin >> c;
            s[i].push_back(c);
        }
    }
    rep(i,k){
        vector<bool> b(s.size(),true);
        sort(all(s));
        rep(j,s.size()-1){
            bool d=true;
            rep(l,k){
                if(s[j+1][l]<s[j][l]){
                    d=false;
                    break;
                }
            }
            if(d) b[j]=false;
        }
        vector<deque<char>> t;
        rep(j,s.size()){
            if(!b[j]) continue;
            s[j].push_back(s[j].front());
            s[j].pop_front();
            t.push_back(s[j]);
        }
        s=t;
    }
    cout << n-s.size() << endl;
}