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You should increase your intake of healthy foods that are high in calories, such as honey, dates, sesame seeds, and halva, and eat plenty of fruit.
Avoid tea, coffee, and other stimulants, and ensure you get adequate sleep. If you follow the above advice and do not achieve the desired weight gain, you should consult a nutritionist for the necessary tests to ensure that your thinness is not due to an underlying medical condition requiring medication.