#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    cin>>n;
    int half=(n+1)/2; 
    for(int i=1;i<=half;i++){
        for(int j=1;j<=half-i;j++){
            cout<<".";
        }
        int len=2*i-1;
        for(int k=1;k<=len;k++){
            if(k==1||k==len){
                cout<<"#";
            }else{
                cout<<".";
            }
        }
        for(int j=1;j<=half-i;j++){
            cout<<".";
        }
        cout<<endl; 
    }
    for(int i=half-1;i>=1;i--){
        for(int j=1;j<=half-i;j++){
            cout<<".";
        }
        int len=2*i-1;
        for(int k=1;k<=len;k++){
            if(k==1||k==len){
                cout<<"#";
            }else{
                cout<<".";
            }
        }
        for(int j=1;j<=half-i;j++){
            cout<<".";
        }
        cout<<endl; 
    }
    return 0;
}

C++ :

#include <algorithm>
#include <cstdio>
using namespace std;
int main() {
	int n, i, j, k;
	scanf("%d", &n);
	k = n / 2;
	for (i = 1; i <= n; i++) {
	for (j = 1; j <= n; j++) {
	if (abs(k - i + 1) + abs(k - j + 1) == k)
	printf("#");
	else
	printf(".");
	}
	printf("\n");
	}
	return 0;
}