Nine Knights Solution:

#include <bits/stdc++.h>
using namespace std;

int main(){
    // #ifndef TESTING
    // freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    // #endif

    char board[5][5];
    for(int i = 0; i < 5; i++){
        for(int j = 0; j < 5; j++){
            cin >> board[i][j];
        }
    }
    bool valid = true;
    int knights = 0;
    for(int i = 0; i < 5; i++){
        for(int j = 0; j < 5; j++){
            if(board[i][j] == 'k'){
                knights++;
                if((i - 1 >= 0 && j - 2 >= 0 && board[i - 1][j - 2] == 'k') || 
                    (i - 1 >= 0 && j + 2 < 5 && board[i - 1][j + 2] == 'k') ||
                    (i + 1 < 5 && j - 2 >= 0 && board[i + 1][j - 2] == 'k') ||
                    (i + 1 < 5 && j + 2 < 5 && board[i + 1][j + 2] == 'k') ||
                    (i - 2 >= 0 && j - 1 >= 0 && board[i - 2][j - 1] == 'k') ||
                    (i + 2 < 5 && j - 1 >= 0 && board[i + 2][j - 1] == 'k') ||
                    (i - 2 >= 0 && j + 1 < 5 && board[i - 2][j + 1] == 'k') ||
                    (i + 2 < 5 && j + 1 < 5 && board[i + 2][j + 1] == 'k')
                ){
                    valid = false;
                }
            }
        }
    }
    
    if(valid && knights == 9){
        cout << "valid" << endl;
    } else {
        cout << "invalid" << endl;
    }

    return 0;
}