C :

int main(int argc, char* argv[])
{
	double s;
int i;
s=0;
for(i=1;i<=100;i++)
{
	if(i%2==0)s=s-1.0/i;
   else s=s+1.0/i;
}
  printf("%.8f\n",s);
	return 0;
}

C++ :

#include<bits/stdc++.h>
using namespace std;
double s;
int main()
{
	for(int i=1;i<=100;i++)
	{
		if(i%2==0)s=s-(1.00000000/i);
		else s=s+(1.00000000/i);
	}
	printf("%.8f",s);
	return 0;
}