public class Main{

public static void main(String[]args){
    float x = 0;
    for(int i = 1; i<1000; i++){
        float temp = 2*i-1;
        float a = 1/temp;
        if(i%2 == 1){
            x += a;
        }else{
            x -= a;
        }
    }
    x *= 4;
    System.out.println(x);
}

}