#!/usr/bin/env python3
# %%
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
from math import pi

# %%
n = int(readline())
m = map(float, read().split())
XA = zip(m, m)

# %%
coef = 0
for x, a in XA:
    coef += a

# %%
print(coef * pi)