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

#define fst first
#define snd second
#define mp make_pair
#define rep(i,a,n) for(int i=(a);i<(n);++i)
#define per(i,a,n) for(int i=(n)-1;i>=(a);--i)
#define all(x) (x).begin(),(x).end()

template <typename T1, typename T2>
bool umin(T1& x, const T2& y){return x>y ? x=y,true : false;}

template <typename T1, typename T2>
bool umax(T1& x, const T2& y){return x<y ? x=y,true : false;}

typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;

const int N = (int)1e6+3;
const int mod = (int)1e9+7;
const int INF = (int)1e9+17;
const ll LLINF = (ll)1e18+17;


int n, m;
int res;
int a[N];
int d[N+N];
bool mark[N+N];

void solve() {
    priority_queue<pii, vector<pii>, greater<pii> > q;
    fill(d, d+N+N, INF);
    d[a[0]] = 0;
    q.push(mp(0, a[0]));
    while (!q.empty()) {
        int v = q.top().snd;
        int dist = q.top().fst;
//        cerr << v << ' ' << dist << endl;
        q.pop();
        if (dist != d[v]) continue;
//        cerr << "here" << endl;
        if (mark[v]) {
            res += dist;
            d[v] = dist = 0;
        }
        rep(i, 0, n) {
            int to = v ^ (1<<i);
//            cerr << to << ' ' << d[to] << endl;
            if (umin(d[to], d[v]+1)) {
                q.push(mp(d[to], to));
            }
        }
    }
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr), cout.tie(nullptr);
#ifdef LOCAL
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif

    //!slow
    cin >> n >> m;
    rep(i, 0, m) {
        string s;
        cin >> s;
        rep(j, 0, n) a[i] |= ((s[j]=='R')<<j);
//        cerr << a[i] << '\n';
        mark[a[i]] = true;
    }
    solve();
    cout << res << endl;

    return 0;
}