#Trace the program

def main():
	a= 6713
	c=0
	d=0
	while(a>0):
		k=a%10
		if(k%3!=0):
			c=c+1
			d=d+k
		a=a//10
	if(c>0):
		print(d/c)
	else:
		print("error")
main()