#include <stdio.h>

char a[21][21], sp;
int n, i, j;
int unu, rez;

int main()
{
	FILE *f = fopen ("copii.in","r");
	FILE *g = fopen ("copii.out","w");
	fscanf (f,"%d", &n);
	for (i=1; i<=n; ++i)
	{
		fscanf (f,"%c", &sp);
		for (j=1; j<=n; ++j)
		{
			fscanf (f,"%c", &a[i][j]);
			if (a[i][j] == '1')
				unu ++;
		}
	}
	fclose(f);
	
	rez = (int) (unu+1) / 2;
	
	fprintf (g,"%d", rez);
	
	fclose(g);
	return 0;
}