500
"They wrapped the message in Doon's shirt and put a rock inside it [...] With all her strength, she cast the message into the darkness, and they watched as it plunged down and down. Mrs. Murdo, walking even more briskly than usual to keep her spirits up, was crossing Harken Square when something fell to the pavement just in front of her with a terrific thump. How extraordinary, she thought, bending to pick it up. It was a sort of bundle. She began to untie it."
How did Mrs. Murdo get Lina and Doon's message?