DFS

#include <iostream>

using namespace std;

const int N = 1010;

int n;
char a[N][N];
int vis[N][N] = {0};
const int dx[] = {0, -1, 0, 1};
const int dy[] = {-1, 0, 1, 0};
int flag;

void dfs(int x, int y)
{
    vis[x][y] = 1;
    if (a[x][y + 1] == '#' && a[x][y - 1] == '#' && a[x + 1][y] == '#' && a[x - 1][y] == '#') flag = 1;
    for (int i = 0; i < 4; i ++)
    {
        int nx = x + dx[i], ny = y + dy[i];
        if (vis[nx][ny] == 0 && a[nx][ny] == '#') dfs(nx, ny);
    }
}

int main()
{
    cin >> n;
    for (int i = 1; i <= n; i ++)
        for (int j = 1; j <= n; j ++)
            cin >> a[i][j];

    int ans = 0;
    for (int i = 1;i <= n; i ++)
        for (int j = 1; j <= n; j ++)
            if (a[i][j] == '#' && vis[i][j] == 0)
            {
                flag = 0;
                dfs(i, j);
                if (flag == 0) ans ++;
            }

    cout << ans << endl;

    return 0;
}