As I understand the jeep leaving the beginning of the road can take an infinite amount of fuel - M liters in the fuel tank and the rest in the driver's mouth. Somewhere on the way we can leave some fuel. After that we can return and take this fuel. But why can't we fill up the tank after M kilometers? We have the fuel in the driver's mouth!!!

Please, anybody who understand this problem explain why I am wrong.

Oh, I'm stupid.
I understood the problem later. The jeep can carry only M liters. So, we can take M liters in the beginning, go on distance X, leave Y liters there and return back. So, 2 * X + Y <= M. Next time, we can take these Y liters and actually we would be able to carry M + Y liters.