#include <stdio.h>
int main()
{
    int n,m;
    scanf("%d %d",&n,&m);
    if(m<0 || n<0){
        printf("-1");
        return 0;
    }
    int matrix[m][n];
    for(int i=0;i<m;i++){
        for(int j=0;i<n;j++){
            scanf("%d",&matrix[i][j]);
        }
    }
    int top =0,bottom =m-1;
    int left=0,right =n-1;
    while(top<bottom && left <=right){
        for(int i = left;i<=right;i++)
            printf("%d",matrix[top][i]);
            top++;
        for(int i =top;i<=bottom;i++)
            printf("%d",matrix[i][right]);
            right--;
        
        if(top <=bottom){
            for(int i=right;i>=left;i--)
                printf("%d",matrix[bottom][i]);
            bottom--;
            }
            
        if(left<=right){
            for(int i=bottom;i>=top;i--)
                printf("%d",matrix[i][left]);
            left++;
            
        }
            
    
    return 0;
}