#include<bits/stdc++.h> using namespace std; int main() { double s=0; for(int i=1;; i+=2) { double t=1.0/i; if(fabs(t)<1e-6) break; if(i%4==1) s+=t; else s-=t; } cout<<"PI="<<fixed<<setprecision(8)<<4*s<<endl; return 0; }