#include <stdio.h>
 
int main(void)
{
    double sum = 0;
    double an = 1;
    int n = 20;
    int k;

    for( k = 0; k < n; k++ )
    {
        sum += an;
        an = -an/2;
    }
    printf("n =%5d\tsum = %f\n", n, sum);
    return 0;
}

n =   20	sum = 0.666666