Can someone help me cut this brown area, so it is only inside the dashed lines?

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Code, edit and compile here:
\documentclass{memoir}\usepackage{tikz}\begin{document}\begin{tikzpicture}\draw[style=help lines] (-0.5,-0.2) grid (8,6.5);\centering\clip(-0.5,-0.2) rectangle (8,6.5);\draw[black,very thick,-latex] (-0.2,0) -- (7,0) node[right] {$x_1$};\draw[black,very thick,-latex] (0,-0.2) -- (0,6) node[left] {$x_2$};%Line :\draw [dashed,thin] plot [smooth, tension=1] coordinates {(0.2,5) (3.15,5.5) (6.5,5)};\draw [dashed,thin] plot [smooth, tension=1] coordinates {(5.7,5.7) (5.5,3) (4.5,0.1)};\draw [dashed,thin] plot [smooth, tension=1] coordinates {(-0.2,3) (2.65,1.1) (5.5,0.4)};\draw [dashdotted,thin] plot [smooth, tension=1] coordinates {(0.5,0) (0.5,6.2)};\draw [dashdotted,thin] plot [smooth, tension=1] coordinates {(0,1) (8,1)};\fill[color=brown, opacity=0.3] (0.5,1) rectangle (6,6) ;%Text :\draw[black,ultra thin] (0.5,4) -- (2,6) node[above] {Side constraint};\draw[black,ultra thin] (5.3,2) -- (5.8,1.5) node[right,text width=2cm] {Behaviour constraint};\draw[black, ultra thin] (3,3.75) -- (3,3.75) node[below] {Feasible region};\end{tikzpicture}\end{document}