// {{{ by shik
// #pragma GCC optimize("O3")
#include <bits/stdc++.h>
#include <unistd.h>
#define SZ(x) ((int)(x).size())
#define ALL(x) begin(x),end(x)
#define REP(i,n) for ( int i=0; i<int(n); i++ )
#define REP1(i,a,b) for ( int i=(a); i<=int(b); i++ )
#define FOR(it,c) for ( auto it=(c).begin(); it!=(c).end(); it++ )
#define MP make_pair
#define PB push_back
using namespace std;
typedef long long LL;
typedef pair<int,int> PII;
typedef vector<int> VI;

#ifdef SHIK
template<typename T>
void _dump( const char* s, T&& head ) { cerr<<s<<"="<<head<<endl; }

template<typename T, typename... Args>
void _dump( const char* s, T&& head, Args&&... tail ) {
    int c=0;
    while ( *s!=',' || c!=0 ) {
        if ( *s=='(' || *s=='[' || *s=='{' ) c++;
        if ( *s==')' || *s==']' || *s=='}' ) c--;
        cerr<<*s++;
    }
    cerr<<"="<<head<<", ";
    _dump(s+1,tail...);
}

#define dump(...) do { \
    fprintf(stderr, "%s:%d - ", __PRETTY_FUNCTION__, __LINE__); \
    _dump(#__VA_ARGS__, __VA_ARGS__); \
} while (0)

template<typename Iter>
ostream& _out( ostream &s, Iter b, Iter e ) {
    s<<"[";
    for ( auto it=b; it!=e; it++ ) s<<(it==b?"":" ")<<*it;
    s<<"]";
    return s;
}

template<typename A, typename B>
ostream& operator <<( ostream &s, const pair<A,B> &p ) { return s<<"("<<p.first<<","<<p.second<<")"; }
template<typename T>
ostream& operator <<( ostream &s, const vector<T> &c ) { return _out(s,ALL(c)); }
template<typename T, size_t N>
ostream& operator <<( ostream &s, const array<T,N> &c ) { return _out(s,ALL(c)); }
template<typename T>
ostream& operator <<( ostream &s, const set<T> &c ) { return _out(s,ALL(c)); }
template<typename A, typename B>
ostream& operator <<( ostream &s, const map<A,B> &c ) { return _out(s,ALL(c)); }
#else
#define dump(...)
#endif

template<typename T>
void _R( T &x ) { cin>>x; }
void _R( int &x ) { scanf("%d",&x); }
void _R( long long &x ) { scanf("%" PRId64,&x); }
void _R( double &x ) { scanf("%lf",&x); }
void _R( char &x ) { scanf(" %c",&x); }
void _R( char *x ) { scanf("%s",x); }

void R() {}
template<typename T, typename... U>
void R( T& head, U&... tail ) {
    _R(head);
    R(tail...);
}

template<typename T>
void _W( const T &x ) { cout<<x; }
void _W( const int &x ) { printf("%d",x); }
template<typename T>
void _W( const vector<T> &x ) {
    for ( auto i=x.cbegin(); i!=x.cend(); i++ ) {
        if ( i!=x.cbegin() ) putchar(' ');
        _W(*i);
    }
}

void W() {}
template<typename T, typename... U>
void W( const T& head, const U&... tail ) {
    _W(head);
    putchar(sizeof...(tail)?' ':'\n');
    W(tail...);
}

#ifdef SHIK
#define FILEIO(...)
#else
#define FILEIO(name) do {\
    freopen(name ".in","r",stdin); \
    freopen(name ".out","w",stdout); \
} while (0)
#endif

// }}}

inline void chkmin( int &a, int b ) {
    if ( b<a ) a=b;
}

const int N=314;
const int INF=1e8;

int w[N][N],dp[N][N];
int solve( int n, char a[], char b[] ) {
    memset(w,0,sizeof(w));
    REP(i,n+1) REP(j,n+1) dp[i][j]=INF;
    dp[0][0]=0;
    REP(i,n) REP(j,n) {
        int me=dp[i][j];
        int d=i-j;
        if ( i==0 || j==0 ) {
            REP(k,n-i) if ( k+d>=0 && k+d<n && a[k]==b[k+d] ) w[i][j]++;
        } else {
            w[i][j]=w[i-1][j-1]-(a[n-i]==b[n-j]);
        }
        me+=w[i][j];
        chkmin(dp[i+1][j],me);
        chkmin(dp[i][j+1],me);
    }
    int ans=INF;
    REP(i,n+1) chkmin(ans,dp[i][n]);
    REP(i,n+1) chkmin(ans,dp[n][i]);
    return ans;
}

int n,m;
char a[N][N];
int main() {
    R(m,n);
    REP(i,m) REP(j,n) R(a[j][i]);
    LL ans=0;
    REP(i,n-1) ans+=solve(m,a[i],a[i+1]);
    W(ans);
    return 0;
}


Measured memory_bytes: 1152