#include<bits/stdc++.h>
#define rep(i, n) for(ll i = 0; i < ll(n); i++)
#define rep2(i, s, n) for(ll i = ll(s); i < ll(n); i++)
#define all(a) a.begin(),a.end()
#define pb push_back
#define eb emplace_back
using namespace std;
using ll = long long;
using P = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vp = vector<P>;
using vvp = vector<vp>;
using vs = vector<string>;
const int inf = 1001001001;
const ll linf = 1001001001001001001;
template<class T>
bool chmin(T &a, T b) {
	return a > b ? a = b, true : false;
}
template<class T>
bool chmax(T &a, T b) {
	return a < b ? a = b, true : false;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n,m;
	cin >> n >> m;
	vs s(n);
	rep(i,n) cin >> s[i];
	int ans = 0;
	rep(j, m) {
		vi v(26);
		rep(i, n) ++v[s[i][j]-'a'];
		ans += n- *max_element(all(v));
	}
	cout << ans << endl;
}