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