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Lights of four colors (red, blue, green, yellow) are hung on a ladder. On every step of the ladder there are two lights. If one of the lights is red, the other light on that step will always be blue. If one of the lights on a step is green, the other light on that step will always be yellow. Which of the following statements is not necessarily correct?

- The number of red lights is equal to the number of blue lights
- The number of green lights is equal to the number of yellow lights
- The sum of the red and green lights is equal to the sum of the yellow and blue lights
- The sum of the red and blue lights is equal to the sum of the green and yellow lights