Editorial for COCI '06 Contest 2 #1 R2

Remember to use this editorial only when stuck, and not to copy-paste code from it. Please be respectful to the problem author and editorialist.
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By definition, the arithmetic mean is $S=\frac{R_1+R_2} 2$ $S = \frac{R_{1} + R_{2}}{2}$ . Therefore, $R_2=2S-R_1$ $R_{2} = 2 S - R_{1}$ .