diff --git a/src/9/3/P3660.cpp b/src/9/3/P3660.cpp
new file mode 100644
index 0000000..e752dd5
--- /dev/null
+++ b/src/9/3/P3660.cpp
@@ -0,0 +1,55 @@
+#include <cstdint>
+#include <iostream>
+#include <istream>
+#include <vector>
+using ll = int64_t;
+
+ll n,ans=0;
+std::vector<ll> vis,t,a;
+
+static inline ll lb(const ll n){
+    return n&(-n);
+}
+
+static inline void upd(ll idx,ll add){
+    while(idx<=2*n){ // 注意要*2
+        t[idx]+=add;
+        idx+=lb(idx);
+    }
+}
+
+static inline ll get(ll idx){
+    ll res=0;
+    while(idx){
+        res+=t[idx];
+        idx-=lb(idx);
+    }
+    return res;
+}
+
+int main(){
+    std::iostream::sync_with_stdio(false);
+    std::cin.tie(nullptr);
+
+    std::cin>>n;
+    a.resize(n*2+1);
+    vis.resize(n+1);
+    t.resize(n*2+1);
+
+    for(ll i=1;i<=n*2;i++){
+        std::cin>>a[i];
+    }
+
+    for(ll i=1;i<=n*2;i++){
+        if(vis[a[i]]){
+            ll nans=get(i)-get(vis[a[i]]);
+            ans+=nans;
+            // printf("%lld += %lld\n",i,nans);
+            upd(vis[a[i]], -1);
+        }else{
+            upd(i, 1);
+            vis[a[i]]=i;
+        }
+    }
+    std::cout<<ans<<"\n";
+}
\ No newline at end of file