The term "elevation" is often used to describe the height of a place (such as a city, a mountain, or a valley) compared to sea level. For example, the city of Houston, Texas has an elevation of 105 feet. The surface of the sea has an elevation of 0 feet. Some places are below sea level, so their elevations are negative values. Two towns have different elevations, but when the elevations are used as inputs of , they both produce an output of 25. Why do they produce the same output?