#include<iostream>

#include<cstdio>

#include<string>

#include<cstring>

#include<algorithm>

using namespace std;

const int maxn = ;

int n,m,ans;

char s[maxn][maxn];

bool ok;

bool vis[maxn];

int main(){

    freopen("evensgn.in","r",stdin);

    freopen("evensgn.out","w",stdout);

    cin>>n>>m;

    for(int i = ;i <= n;i++){

            scanf("%s",s[i]+);

    }

    memset(vis,true,sizeof(vis));

    for(int i = ;i <= m;i++){

        ok = true;

        for(int j = ;j <= n;j++){

            if(vis[j]&& s[j][i] < s[j-][i]){

                ans++;

                ok = false;

                break;

            }

        }

        if(ok){

            for(int j = ;j <= n;j++){

                if(s[j][i] != s[j-][i]) vis[j] = false;

            }

        }

    }

    cout<<ans;

    return ;

}

#include<cstdio>

#include<iostream>

#include<algorithm>

#include<cstring>

#include<cmath>

#include<vector>

#include<queue>

#include<map>

#include<set>

#include<stack>

#include<cstdlib>

#include<string>

#include<bitset>

#define INF 100000000

#define fi first

#define se second

using namespace std;

typedef long long LL;

typedef pair<int,int> pii;

char s[][];

int rk[],trk[];

int main()

{

    int n,m,i,j,ans=,flag;

    freopen("evensgn.in","r",stdin);

    freopen("evensgn.out","w",stdout);

    cin>>n>>m;

    for(i=;i<=n;i++)

        scanf("%s",s[i]+);

    for(j=;j<=m;j++)

    {

        flag=;

        trk[]=;

        for(i=;i<n;i++)

        {

            if(rk[i]<rk[i+])

            {

                trk[i+]=trk[i]+;

                continue;

            }

            else

            {

                if(s[i][j]>s[i+][j])

                {

                    ans++;

                    //printf("j=%d i=%d\n",j,i);

                    flag=;

                    break;

                }

                else if(s[i][j]<s[i+][j])

                    trk[i+]=trk[i]+;

                else trk[i+]=trk[i];

            }

        }

        if(!flag)

            for(i=;i<=n;i++)

                rk[i]=trk[i];

    }

    cout<<ans<<endl;

    return ;

}

// davidlee1999WTK 2014/

// srO myk Orz

//ios::sync_with_stdio(false);