I'm assuming that the lines are finite since it says first and last trees. Call the sides U and D (up and down), and let the trees starting from the end one on the left on U be a_1,a_2,...,a_n and D be b_1,b_2,...,b_n. Then (numbers on a_1 vs. b_1) a_1+a_2=b_1+b_2; a_1+a_2+a_3=b_1+b_2+b_3 implies a_3=b_3, then comparing the numbers on trees a_3 and b_3 we get a_4=b_4, ..., a_{n-1}=b_{n-1}, a_n=b_n; going back to comparing the numbers on tree a_3, we get a_2+a_3+a_4=b_2+b_3+b_4 implies a_2=b_2, comparing on tree a_2 and b_2 we get a_1=b_1 so done