#pragma GCC optimize("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("avx,avx2,fma")

#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define ld long double


const int N = 2e5 + 5, mod = 1e9 + 7;



int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int n, m;
    cin >> n >> m;
    vector<string> a(n);
    for(int i = 0; i < n; i++) cin >> a[i];

    int ans = 0;
    for(int i = 0; i < m; i++) {
        vector<int> freq(26, 0);
        for(int j = 0; j < n; j++) {
            freq[a[j][i] - 'a']++;
        }
        ans += n - *max_element(freq.begin(), freq.end());
    }
    cout << ans;

    return 0;
}