#include <stdio.h>

int main() #include <stdio.h>

int main() {
    int m, n, a[100][100];
    scanf("%d %d", &m, &n);

    if (m < 0 || n < 0) {
        printf("-1");
        return 0;
    }

    char s[200];
    for (int i = 0; i < m; i++) {
        scanf("%s", s);
        for (int j = 0; j < n; j++)
            a[i][j] = s[j] - '0';
    }

    int t = 0, b = m - 1, l = 0, r = n - 1;

    while (t <= b && l <= r) {
        for (int i = l; i <= r; i++) printf("%d", a[t][i]); t++;
        for (int i = t; i <= b; i++) printf("%d", a[i][r]); r--;
        if (t <= b) for (int i = r; i >= l; i--) printf("%d", a[b][i]); b--;
        if (l <= r) for (int i = b; i >= t; i--) printf("%d", a[i][l]); l++;
    }

    return 0;
}