#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using i64 = long long;
#define rep(i,n) for(i64 i=0; i<i64(n); i++)
const i64 INF = 1001001001001001001;
template<typename A> void chmin(A& l, const A& r){ if(r < l) l = r; }
template<typename A> void chmax(A& l, const A& r){ if(l < r) l = r; }
using namespace std;

void testcase(){
    i64 H, W; cin >> H >> W;
    vector<string> A(H); rep(y,H) cin >> A[y];
    vector<pair<i64,i64>> nx;
    rep(a,H) rep(b,H) if(a < b){
        i64 fl = 0;
        rep(i,W){
            if(A[a][i] < A[b][i]) fl |= 1;
            if(A[a][i] > A[b][i]) fl |= 2;
        }
        if(fl == 3) continue;
        if(fl == 2) nx.push_back({a,b});
        else nx.push_back({b,a});
    }
    vector<i64> dp(H, H-1);
    rep(i,H){
        vector<i64> nxdp(H, H-1);
        for(auto [a,b] : nx) chmin(nxdp[b], dp[a] - 1);
        swap(dp, nxdp);
    }
    cout << *min_element(dp.begin(), dp.end()) << "\n";
}

int main(){
    ios::sync_with_stdio(false); cin.tie(nullptr);
    testcase();
    return 0;
}