#include <stdio.h>

    #define F float
    #define c(x,n) 1+g(1,n,x,1,1,1)
    F g(F i,F n,F x,F s,F p,F f){s=-s;p*=x*x;f*=i;return i<n?g(i+1,n,x,s,p,f)+s/2*p/f:0;}

int main(void)
{
	printf("%f", c(0.5,2));
	return 0;
}