public static void main(String[] args) {

String[] original = getInput();

if(original.length != 0) examineThreeParts(original, original[0].length());

}

private static String[] getInput() {

Scanner sc = new Scanner(System.in);

int n = sc.nextInt();

sc.nextLine();

String s = sc.nextLine();

if(s.charAt(0) == s.charAt(1)) {

sc.close();

System.out.print("NO");

return new String[0];

}

String[] original = new String[n];

original[0] = s;

int index = 1;

while(sc.hasNextLine()) {

original[index] = sc.nextLine();

index++;

}

sc.close();

return original;

}

private static void examineThreeParts(String[] original, int n) {

char x = original[0].charAt(0), y = original[0].charAt(1);

// the first n / 2 - 1 strings to be examined

for(int i = 0, j = n - 1; i != j; i++, j--)

if(!examineOneString(original[i], i, j, x, y)) return ;

// the middle string to be examined

for(int i = 0; i < n; i++) {

char c = original[n / 2].charAt(i);

if(i == n / 2) {

if( c != x) {

System.out.print("NO");

return ;

}

} else {

if(c != y) {

System.out.print("NO");

return ;

}

}

}

// the last n / 2 - 1 strings to be examined

for(int i = n / 2 - 1, j = n / 2 + 1; i >= 0; i--, j++)

if(!examineOneString(original[j], i, j, x, y)) return ;

// all clear

System.out.print("YES");

}

private static boolean examineOneString(String string, int i, int j, char x, char y) {

for(int k = 0; k < string.length(); k++) {

char c = string.charAt(k);

if(k == i || k == j) {

if(c != x) {

System.out.print("NO");

return false;

}

} else {

if(c != y) {

System.out.print("NO");

return false;

}

}

}

return true;

}

}