使用泰勒級數公式計算圓周率

根據泰勒級數關系式：pi / 4 = 1 - 1/3 + 1/5 - 1/7 + ..... + (-1)^k (1 / (2k+1) ) + ....

求圓周率的值，當最后一項的值小於給定的閾值時結束

threshold = eval(input())
pi4 = k = 0 
f = 1
while abs(1 / (2 * k + 1)) >= threshold:  
    pi4 = pi4 + f * 1 / (2 * k + 1)  
    k = k + 1
    f = -f
print("{:.6f}".format(pi4*4))