b554

/* b554            */
/* AC (2ms, 116KB) */
#include <cstdio>
#define max(x,y) ((x)>(y)?(x):(y))

using namespace std;

int n, ans;
bool arr[7][7], vis[7][7]{};

void dfs(int i, int j, int step)
{
    if(i < 0 || j < 0 || i >= n || j >= n)
    {
        ans = max(ans, step - 1);
        return;
    }
    if(!arr[i][j] || vis[i][j])
        ans = max(ans,step - 1);
    else
    {
        vis[i][j] = true;
        dfs(i - 1, j, step + 1);
        dfs(i + 1, j, step + 1);
        dfs(i, j - 1, step + 1);
        dfs(i, j + 1, step + 1);
        vis[i][j] = false;
    }
}

int main()
{
    ans = -1;

    scanf("%d", &n); getchar();

    for(int i = 0, t; i < n; getchar(), i++)
        for(int j = 0; j < n; arr[i][j] = t - '0', j++)
            t = getchar();

    dfs(0, 0, 1);

    printf("%d", ans);

    return 0;
}