#include <algorithm>
#include <iostream>
using namespace std;

int arr[30][105] = {0};

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

    int n, m;
    cin >> n >> m;

    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {
            char c;
            cin >> c;
            arr[c - 'a'][j] += 1;
        }
    }

    int ans = n * m;
    for (int j = 0; j < m; ++j) {
        int maxx = 0;
        for (int i = 0; i < 'z' - 'a'; ++i) {
            maxx = max(maxx, arr[i][j]);
        }
        ans -= maxx;
    }

    cout << ans << "\n";

    return 0;
}