Create overflow.md

advanced_expert/vulnerabilities/overflow.md

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+## overflow of numbers
+
+<br>
+
+
+<br>
+
+----
+
+### unchecked math
+
+<br>
+
+* overflow and underflow of numbers in solidity 0.8 throw an error. this can be disabled with `unchecked`.
+* disabling overflow / underflow check saves gas.
+
+<br>
+
+```
+contract UncheckedMath {
+    function add(uint x, uint y) external pure returns (uint) {
+        // 22291 gas
+        // return x + y;
+
+        // 22103 gas
+        unchecked {
+            return x + y;
+        }
+    }
+
+    function sub(uint x, uint y) external pure returns (uint) {
+        // 22329 gas
+        // return x - y;
+
+        // 22147 gas
+        unchecked {
+            return x - y;
+        }
+    }
+
+    function sumOfCubes(uint x, uint y) external pure returns (uint) {
+        // Wrap complex math logic inside unchecked
+        unchecked {
+            uint x3 = x * x * x;
+            uint y3 = y * y * y;
+
+            return x3 + y3;
+        }
+    }
+}
+
+```
+
+<br>