Level 19 looks mathematically impossible: you have two ways at start, desert at your left and snow at your right. Desert will turn into 5 then 6, blocking path after first passage so you have to finish the right side first... But the snow tile at the end of the right path is a 2 and you have to leave it 2 times for each the upper and lower ways... But then you have to go back but as the tile is a snow 2, it has collapsed.
Beside that, the game is great! Nice puzzles, enjoyable concept and cute frog :)