import math
a = int(input())
temp = 3.14
val = str(2*temp*a)
st=""
ind=0
for j in range(len(val)):
	if(val[j]=="."):
		ind=j
		st+="."
		break
	else:
		st+=val[j]
st+=val[ind+1]
st+=val[ind+2]

print(st)
