#include "mpi.h"
#include <stdio.h>

int
main(int argc, char *argv[])
{
    int i, step, n, m, myrank, np;
    MPI_Status status;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
    MPI_Comm_size(MPI_COMM_WORLD, &np);

    n = 1;

    step = 1;
    while (step < np) {
	if ((myrank % step) != 0) break /*continue*/;
	i = myrank / step;
	if ((i % 2) == 0) {
	    if (myrank + step < np) {
		MPI_Recv(&m, 1, MPI_INT, myrank + step, step, MPI_COMM_WORLD,
			 &status);
		n += m;
	    }
	}
	else {
	    MPI_Send(&n, 1, MPI_INT, myrank - step, step, MPI_COMM_WORLD);
	}
	step *= 2;
    }

    if (myrank == 0)
	printf("result = %d\n", n);

    MPI_Finalize();
    return 0;
}