restructure zeta/** to storage/zeta/**

storage/zeta/cpp/completed/15894.cpp

@@ -0,0 +1,14 @@
+#include <iostream>
+
+using namespace std;
+
+long long solve(long long n) {
+    return 4 * n;
+}
+
+int main() {
+    long long n;
+    cin >> n;
+    cout << solve(n);
+    return 0;
+}