1. The problem is to understand and correctly enter the constant $\pi$ in STACK as an exact numerical constant. 2. The constant $\pi$ is approximately 3.14 but is an irrational number with an infinite decimal expansion. 3. To enter $\pi$ exactly in STACK, you should type either $\text{Pi}$ or $\text{pi}$ (case sensitive). 4. Decimal approximations like 3.14 are not exact and should not be used when an exact answer is required. 5. Therefore, the exact value is entered as $\pi$ represented by $\text{Pi}$ or $\text{pi}$. Final answer: Enter $\pi$ as $\text{pi}$ or $\text{Pi}$ in STACK for an exact value.