A question about $L^p$ spaces

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If $1 \leqslant p_1,p_2 \leqslant \infty$ and $L^{p_1}(\mathbb{R}) \subseteq L^{p_2}(\mathbb{R})$ ,prove that $p_1=p_2$

Can some one give me a hint to solve this?

Thank you in advance!