How do I calculate the Standard Deviation of Portfolio's Return?

Portfolio has 2 assets - A & B

Weight of Asset A - 20%

Expected Return of Asset A - 5%

Weighted Average Return = 0.2 X 5% = 1%

Weight of Asset B - 80%

Expected Return of Asset B - 3%

Weighted Average Return = 0.8 X 3% = 2.4%

The portfolio return is 1% + 2.4% = 3.4%

How do I calculate the Standard Deviation of the above portfolio return?

Thank you very much.