#include <stdio.h>

#define N 1000000
#define REPS 1000

double x[N];

/* sum using Kahan summation */
double sumk(int n, double *x)
{
    double s, t, c, y;
    int i;

    s = x[0];
    c = 0.0;
    for (i = 1; i < N; i++) {
	y = x[i] - c;
	t = s + y;
	c = (t - s) - y;
	s = t;
    }
    return s;
}    

int main()
{
    double s;
    int i, j;

    for (i = 0; i < N; i++)
	x[i] = i + 1;

    for (j = 0; j < REPS; j++)
	s = sumk(N, x);

    printf("sum = %f\n", s);
    return 0;
}