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lucrez eu acu`
 
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Fie $N$ ({$N$} ≤ $1000$) un numar natural. Numim sir _2-3-monoton_ de lungine $N$ un sir $S{~1~}$, $S{~2~}$, $S{~3~}$, ... $S{~n~}$ format din $N$ elemente ale multimii ${1, 2, ... N}$ care verifica urmatoarele doua relatii:

* $S{~i~}$ < $S{~i+2~}$, oricare ar fi $1$ ≤ $i$ ≤ $N-2$

* $S{~i~}$ ≤ $S{~i+2~}$, oricare ar fi $1$ ≤ $i$ ≤ $N-2$

* $S{~i~}$ < $S{~i+3~}$, oricare ar fi $1$ ≤ $i$ ≤ $N-3$
h2. Cerinta

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