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**emakarov** Write numbers from 0 to, say, 10 in a row. The minimum value for n in the equation is 4 because there is an index n - 4. So, starting from n = 4 calculate the indices occurring in the right-hand side of the equation, i.e., n - 1, n - 3 and n - 4. If any of these numbers are not circled yet (and initially none of the numbers from 0 to 10 are circled), draw a circle around them and also write that number in a separate place. Finally, for the same value of n draw a circle around n + 1 (no need to write n + 1 separately). The newly circled numbers have to be provided to start the computation. Continue doing this for n = 5, 6, etc., and in the end see which numbers you have written in that separate place.