Add $1_2 + 11_2 + 111_2 + 1111_2.$ Express your answer in base 2.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Add $1_2 + 11_2 + 111_2 + 1111_2.$ Express your answer in base 2.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
11010_2
Add $1_2 + 11_2 + 111_2 + 1111_2.$ Express your answer in base 2.
train