#ifdef NACHIA
#define _GLIBCXX_DEBUG
#else
#define NDEBUG
#endif
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

namespace nachia{

template<
    class S,
    S op(S l, S r)
>
struct Segtree {
private:
    int N;
    std::vector<S> A;
    int xN;

    void mergev(int i){
        if(i < N) A[i] = op(A[i*2], A[i*2+1]);
    }

    template<class E>
    int minLeft2(int r, E cmp, int a = 0, int b = 0, int i = -1) const {
        static S x;
        if(i == -1){ a=0; b=N; i=1; x=A[0]; }
        if(r <= a) return a;
        if(b <= r){
            S nx = op(A[i], x);
            if(cmp(nx)){ x = nx; return a; }
        }
        if(b - a == 1) return b;
        int q = minLeft2(r, cmp, (a+b)/2, b, i*2+1);
        if(q > (a+b)/2) return q;
        return minLeft2(r, cmp, a, (a+b)/2, i*2);
    }
    
    template<class E>
    int maxRight2(int l, E cmp, int a = 0, int b = 0, int i = -1) const {
        static S x;
        if(i == -1){ a=0; b=N; i=1; x=A[0]; }
        if(b <= l) return b;
        if(l <= a){
            S nx = op(x, A[i]);
            if(cmp(nx)){ x = nx; return b; }
        }
        if(b - a == 1) return a;
        int q = maxRight2(l, cmp, a, (a+b)/2, i*2);
        if(q < (a+b)/2) return q;
        return maxRight2(l, cmp, (a+b)/2, b, i*2+1);
    }

public:
    Segtree() : N(0) {}
    Segtree(int n, S e) : xN(n) {
        N = 1; while (N < n) N *= 2;
        A.assign(N * 2, e);
    }
    Segtree(const std::vector<S>& a, S e) : Segtree(a.size(), e){
        for(int i=0; i<(int)a.size(); i++) A[i + N] = a[i];
        for(int i=N-1; i>=1; i--) mergev(i);
    }

    S getE() const { return A[0]; }

    void set(int p, S x){
        p += N; A[p] = x;
        for(int d=1; (1<<d)<=N; d++) mergev(p>>d);
    }

    S get(int p) const { return A[N+p]; }

    S prod(int l, int r) const {
        l += N; r += N;
        S ql = A[0], qr = A[0];
        while(l<r){
            if(l&1) ql = op(ql, A[l++]);
            if(r&1) qr = op(A[--r], qr);
            l /= 2;
            r /= 2;
        }
        return op(ql, qr);
    }

    S allProd() const { return A[1]; }

    // bool cmp(S)
    template<class E>
    int minLeft(int r, E cmp) const {
        return minLeft2(r, cmp);
    }

    // bool cmp(S)
    template<class E>
    int maxRight(int l, E cmp) const {
        int x = maxRight2(l, cmp);
        return x > xN ? xN : x;
    }
};

} // namespace nachia
#include <functional>
#include <utility>

namespace nachia {
    template<class T, class Cmp = std::less<T>>
    struct PointSetRangeMin{
    private:
        static T minop(T l, T r){ return std::min(l, r, Cmp()); }
        using Base = Segtree<T, minop>;
        Base base;
        Cmp cmpx;
    public:
        PointSetRangeMin() {}
        PointSetRangeMin(int len, T INF)
            : base(len, INF){}
        PointSetRangeMin(const std::vector<T>& init, T INF)
            : base(init, INF){}
        T min(int l, int r){ return base.prod(l, r); }
        T min(){ return base.allProd(); }
        void set(int pos, T val){ base.set(pos, val); }
        T getinf() const { return base.getE(); }
        void setinf(int pos){ set(pos, getinf()); }
        T get(int pos){ return base.get(pos); }
        void chmin(int pos, T val){ base.set(pos, minop(get(pos), val)); }
        int lBoundLeft(int from, T val){ return base.minLeft(from, [this,val](const T& x){ return cmpx(val, x); }); }
        int uBoundLeft(int from, T val){ return base.minLeft(from, [this,val](const T& x){ return !cmpx(x, val); }); }
        int lBoundRight(int from, T val){ return base.maxRight(from, [this,val](const T& x){ return cmpx(val, x); }); }
        int uBoundRight(int from, T val){ return base.maxRight(from, [this,val](const T& x){ return !cmpx(x, val); }); }
        template<class E>
        int minLeft(int r, E cmp){ return base.minLeft(r, cmp); }
        template<class E>
        int maxRight(int l, E cmp){ return base.maxRight(l, cmp); }
        int argmin(int l, int r){
            auto v = this->min(l, r);
            if(!cmpx(v, getinf())) return -1;
            return lBoundRight(l, v);
        }
    };
} // namespace nachia
using i64 = long long;
using u64 = unsigned 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);
    rep(t,1) testcase();
    return 0;
}