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