first commit

p28/solution.jl

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+
+function solution(n)
+    if n % 2 == 0
+        return -1
+    end
+    if n == 1
+        return 1
+    else
+        sum_n = 4*n^2 - 6*(n-1)
+        return solution(n-2) + sum_n
+    end
+end