Analyze the sentiment of this tweet.
The sentiment of this tweet is negative. The author expresses their disappointment and sadness about missing their friend's birthday due to work obligations.
It's so sad that I have to miss my friend's birthday because of work.
Below is an instruction that describes a task, paired with an input that provides further context. Write a response that appropriately completes the request.

### Instruction:
Analyze the sentiment of this tweet.

### Input:
It's so sad that I have to miss my friend's birthday because of work.

### Response:
The sentiment of this tweet is negative. The author expresses their disappointment and sadness about missing their friend's birthday due to work obligations.