public class Test {
public static void main(String[] aergs) {
int n = 3;
double sum = 1;
for(int i = 0; i < 10000; i++,n+=2) {
if(i % 2 == 0) {
sum -= 1/n;
} else {
sum += 1/n;
}
}
System.out.println(sum);
}
}