BFS

#include <iostream>
#include <queue>
#include <array>
#include <vector>

/*
5 8
....##..
..##.##.
###...##
#.#..###
#..####.
*/

struct Coord {
    int i;
    int j;
};

int CountReachableFrom(std::vector<std::vector<bool>>& field, const Coord& start) {
    const int n = field.size();
    const int m = field.at(0).size();

    std::queue<Coord> queue;
    queue.push(start);
    field[start.i][start.j] = true;

    int count = 1;
    while (!queue.empty()) {
        const Coord current = queue.front();
        queue.pop();

        for (const Coord delta : std::vector<Coord>{{0, 1}, {1, 0}, {0, -1}, {-1, 0}}) {
            const Coord next{current.i + delta.i, current.j + delta.j};
            if (next.i < 0 || next.j < 0 || next.i >= n || next.j >= m) {
                continue;
            }
            if (field[next.i][next.j] == false) {
                field[next.i][next.j] = true;
                queue.push(next);
                ++count;
            }
        }
    }

    return count;
}

int main(int, const char**) {
    int n, m;
    std::cin >> n >> m;
    std::string unused;
    std::getline(std::cin, unused);

    std::vector<std::vector<bool>> field(n, std::vector<bool>(m));
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {
            char ch;
            std::cin >> ch;
            if (ch == '#') {
                field[i][j] = true;
            } else {
                field[i][j] = false;
            }
        }
        std::getline(std::cin, unused);
    }

    int answer = CountReachableFrom(field, {1, 4});
    std::cout << answer << "\n";
    return 0;
}