#include
int main(int argc,char *argv[]){
double pi,t,f;
int i;
printf("Input f(double 0.1>=f>=0.00001)...\nf=");
if(scanf("%lf",&f)!=1 || f<0.00001 || f>0.1){
printf("Input error, exit...\n");
return 0;
}
for(pi=0.0,t=i=1;t>=f;i+=2){
t=1.0/i;
pi += (i+1)%4 ? t : -t;
}
printf("π≈%f\n",4*pi);
return 0;
}