Valmiki Ramayana Ayodhya Kanda Sarga 28 Shloka 20
Original Shlokaनदी निलयना स्सर्पा नदीकुटिलगामिनः। तिषठ्न्त्यावृत्य पन्थानं ततो दुःखतरं वनम्।।.2.28.20।।
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नदी निलयनाः live in rivers, नदीकुटिलगामिनः meandering like rivers, सर्पाः serpents, पन्थानम् the path, आवृत्य having obstructed, तिष्ठन्ति stay, ततः therefore, वनम् life in the forest, दुःखतरम् is full of sufferings.
The serpents that live in and move like meandering rivers always obstruct the paths. Therefore, living in the forest is tremendously difficult.