351B - Jeff and Furik

https://codeforces.com/problemset/submission/351/173713009

Codeforces/351/B/B.cpp

@@ -0,0 +1,43 @@
+#include <iostream>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 3005;
+
+int n, p[N], c[N], ans;
+
+int lowbit(int x) {
+    return x & -x;
+}
+
+void add(int x, int y) {
+    for (; x <= n; x += lowbit(x)) c[x] += y;
+}
+
+int sum(int x) {
+    int res = 0;
+    for (; x; x -= lowbit(x)) res += c[x];
+    return res;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n;
+
+    for (int i = 1; i <= n; i++) {
+        cin >> p[i];
+    }
+
+    for (int i = n; i; i--) {
+        ans += sum(p[i] - 1);
+        add(p[i], 1);
+    }
+
+    cout << (ans * 2 - ans % 2) << endl;
+
+    return 0;
+}