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