I am sorry of this stupid question

I would like to know if it possible to calculate the standard deviation of a difference of means, having the two primary standard deviation and the two primary sample size and the two primary means.

For example:

I have 24 men that at T0 weigh [mean (a)=77.5 Kg SD(b)=1.4 N=24]; after one year the same 24 men weigh [mean (a)=81.4 kg SD(a)= 3.9 N=24]. Hence on average every man increases his weight of 3.9 Kg; but what is the SD of this difference? Is it possible to calculate it with this data?

Please help me, I really need this data.

Thanks