#ifndef Yamada
#define Yamada
#include Yamada __FILE__ Yamada

void solve() {
    int n, m; cin >> n >> m;
    vector<string> v(n);
    for (auto &s : v) cin >> s;
    
    int ans = 0;
    for (int i = 0; i < m; ++i) {
        vector<int> cnt(27, 0);
        for (int j = 0; j < n; ++j) {
            ++cnt[v[j][i] - 'a'];
        }
        int mx = 0;
        for (int j = 0; j < n; ++j) mx = max(mx, cnt[v[j][i] - 'a']);
        ans += n - mx;
    }
    cout << ans << '\n';
}

signed main() {
    IOS();
    int t = 1; // cin >> t;
    for (int i = 1; i <= t; ++i) solve();
    return 0;
}

#else

#include <bits/stdc++.h>
using namespace std;
#define int int64_t
using pii = pair<int,int>;

#define SZ(a) ((int)(a).size())
#define ALL(a) begin(a), end(a)
#define RALL(a) rbegin(a), rend(a)
#define ee emplace
#define eb emplace_back
#define ef emplace_front
#define pb pop_back
#define pf pop_front

#ifdef local
#define IOS() void()
#else
#define IOS() ios_base::sync_with_stdio(0); cin.tie(0);
#endif

#endif