# include<stdio.h>
# define N 3
int main()
{
	int a[N][N],sum=0,t=0;
	double ave;
	for (int i = 0; i < N; i++)
	{
		for (int j = 0; j < N; j++)
		{
			scanf_s("%d",&a[i][j]);
		}
	}
	for (int i = 0; i < N; i++)
	{
		for (int j = 0; j < N; j++)
		{
			if ((i == 0 || i == N - 1) && (j == 0 || j == N - 1))
			{
				sum = sum + a[i][j];
				t++;
			}
		}
	}
	ave = (double)sum / t;
	printf("%lf",ave);
	return 0;
}