#include <algorithm>
#include <deque>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <random>
#include <bits/stdc++.h>

using namespace std;
#define F first
#define S second
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define per(i, a, b) for(int i = b-1; i>=a ; i--)
#define trav(a, x) for(auto& a : x)
#define allin(a , x) for(auto a : x)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<ll> vl;
typedef vector<pii> vpi;
typedef pair<ll,ll> pll;
typedef vector<string> vs;
typedef vector<pll> vpl;
typedef vector<int> vi;

ll cdiv(ll a, ll b) { return a/b+((a^b)>0&&a%b); } // divide a by b rounded up
ll fdiv(ll a, ll b) { return a/b-((a^b)<0&&a%b); } // divide a by b rounded down

#define Unique(v) sort(all(v));v.erase(unique(all(v)),v.end());

const int maxn = 2*100000 + 10;

// const int inf = 1000000000;
// const ll inf = 1000000000000000000LL;

// const ll mod = 1000000007; // 10^9 + 7
// const ll mod = (119 << 23) + 1; // 998244353

int n, m;

string s[maxn];

int cost(vector<int> freq)
{
    int ans = n;

    for(int i = 0 ; i < (int)freq.size() ; i++)
        ans = min( ans , n - freq[i] );

    return ans;
}

void solve(int testcase)
{
    cin >> n >> m;

    for(int i = 1 ; i <= n ; i++)
        cin >> s[i];

    int ans = 0;

    for(int i = 0 ; i < m ; i++)
    {
        vector<int> freq(26, 0);

        for(int j = 1 ; j <= n ; j++)
            freq[ s[j][i] - 'a' ]++;

        ans += cost( freq );
    }

    cout << ans << endl;
}

int main()
{
    ios_base::sync_with_stdio(false);  
    cin.tie(NULL);

    int t = 1;
    // cin >> t;

    for(int testcase = 1 ; testcase <= t ; testcase++)
        solve( testcase );
}