Pi equals the circumference of a circle divided by it's diameter. This value is approximately 3.14159265359, but goes on indefinitely. For most sums 3.14 is acceptable, or just use the pi button on a scientific calculator.
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Negative pi has the same value as pi, but with a minus sign before it. Common approximations to pi are 3.14 (so -pi would be -3.14) and 22/7 (so -pi would be -22/7). They are both approximations: pi (and -pi) can only be expressed as a never-ending decimal.