C :

#include <stdio.h>
#include <math.h>
int main ()
{
int i;
double  s;
s = 0;
for(i=0; i<=100000; i++)
{
s+=pow(-1,i)/(2*i+1);
}
s=4*s;
printf("%.1f",s);
return 0;
}

Java :

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		double sum = 0;
		int flag = 1;
		for (int i = 1; i <= 50; i++)
		{
			sum += 1.0 / (2*i-1) * flag;
			flag *= -1;
		}
		sum *= 4;
		System.out.printf("%.1f\n",sum);
	}

}