def get_pair_st_so(st_id,so_id):
    """
    Get string st<st_id>-so<so_id> (e.g. st0-so0).
    
    Parameters
    ----------
     st_id : int
         station id.
     so_id : int
         source id.
     
    Returns
    -------
     pair_st_so : str
         output.
    """
    return("st"+str(st_id)+"-"+"so"+str(so_id))