def num_region_obs(count_arr, lon_idxs, lat_idxs):
    """
    Given lon/lat indices and array of GOSAT counts, determine number of
    observations in the region as defined by the indices.

    Parameters:
        count_arr (np arr) : array of counts (Mx72x46), M num months
        lon_idxs  (np arr) : array of longitude indices
        lat_idxs  (np arr) : array of latitude indices

    Returns:
        numpy array of counts per month
    """
    return count_arr[
        :,
        lon_idxs[0]:lon_idxs[-1],
        lat_idxs[0]:lat_idxs[-1]
    ].sum(axis=1).sum(axis=1)