cs: add «2491. Divide Players Into Teams of Equal Skill»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/divide-players-into-teams-of-equal-skill.cs

@@ -0,0 +1,18 @@
+public class Solution {
+    public long DividePlayers(int[] skill) {
+        Array.Sort(skill);
+
+        var expectedSkill = skill[0] + skill[skill.Length - 1];
+
+        long chemistry = 0;
+        for (int i = 0, j = skill.Length - 1; i < j; ++i, --j) {
+            if (skill[i] + skill[j] != expectedSkill) {
+                return -1;
+            }
+
+            chemistry += skill[i] * (long)skill[j];
+        }
+
+        return chemistry;
+    }
+}