#include <iostream>
using namespace std;
int main() {
    int n, m;
    cin >> n >> m;
    int cnt = 0;
    char a[n + 5][m + 5];
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cin >> a[i][j];
        }
    }
    bool _1, _2, _3;
    _1 = _2 = _3 = false;
    for (int i = 0; i < n; i++) {
        if ((i + 1) % 3 == 1) {
            _1 = true;
        } else if ((i + 1) % 3 == 2) {
            _2 = true;
        } else if ((i + 1) % 3 == 0) {
            _3 = true;
        }
        for (int j = 0; j < m; j++) {
            if (_1 == true) {
                if (a[i][j] != 'W') {
                    cnt++;
                }
            }
            if (_2 == true) {
                if (a[i][j] != 'B') {
                    cnt++;
                }
            }
            if (_3 == true) {
                if (a[i][j] != 'R') {
                    cnt++;
                }
            }
        }
        _1 = _2 = _3 = false;
    }
    cout << cnt << endl;
    return 0;
}