diff --git a/CMakeLists.txt b/CMakeLists.txt
index f608864..fd4135c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -30,4 +30,5 @@ add_executable(Playground
         csp-backpack/main.cc
         csp-awarding/main.cc
         holiday-plan/main.cc
-        bears-fruit-basket/main.cc)
+        bears-fruit-basket/main.cc
+        pre-sum/main.cc)
diff --git a/insert-sort/main.cc b/insert-sort/main.cc
index f4ffb92..fe9bc19 100644
--- a/insert-sort/main.cc
+++ b/insert-sort/main.cc
@@ -5,7 +5,7 @@ using namespace std;
 const int N = 10005;
 
 pair<int, int> arr[N];
-int b[N];
+int beta[N];
 
 int main() {
     int n, m;
@@ -15,23 +15,23 @@ int main() {
         arr[i].second = i;
     }
     sort(arr + 1, arr + n + 1);
-    for (int i = 1; i <= n; i++) b[arr[i].second] = i;
+    for (int i = 1; i <= n; i++) beta[arr[i].second] = i;
     while (m--) {
         int op, x, y;
         scanf("%d", &op);
         if (op == 1) {
             scanf("%d%d", &x, &y);
-            arr[b[x]].first = y;
+            arr[beta[x]].first = y;
             for (int i = 1; i < n; i++) {
                 if (arr[i] > arr[i + 1]) swap(arr[i], arr[i + 1]);
             }
             for (int i = n - 1; i >= 1; i--) {
                 if (arr[i] > arr[i + 1]) swap(arr[i], arr[i + 1]);
             }
-            for (int i = 1; i <= n; i++) b[arr[i].second] = i;
+            for (int i = 1; i <= n; i++) beta[arr[i].second] = i;
         } else {
             scanf("%d", &x);
-            printf("%d\n", b[x]);
+            printf("%d\n", beta[x]);
         }
     }
     return 0;
diff --git a/pre-sum/data.in b/pre-sum/data.in
new file mode 100644
index 0000000..31966e2
--- /dev/null
+++ b/pre-sum/data.in
@@ -0,0 +1,3 @@
+3
+2 3 4
+3 4 5
diff --git a/pre-sum/main.cc b/pre-sum/main.cc
new file mode 100644
index 0000000..a987b08
--- /dev/null
+++ b/pre-sum/main.cc
@@ -0,0 +1,30 @@
+#include <iostream>
+
+using namespace std;
+long long alpha[500005], beta[500005];
+long long pre_alpha[500005], pre_beta[500005];
+long long sum_alpha, sum_beta;
+long long ans;
+
+const int mod = 1000000007;
+
+int main() {
+    int n;
+    cin >> n;
+    for (int i = 1; i <= n; ++i) {
+        cin >> alpha[i];
+        pre_alpha[i] = (pre_alpha[i - 1] + alpha[i]) % mod;
+    }
+    for (int i = 1; i <= n; ++i) {
+        cin >> beta[i];
+        pre_beta[i] = (pre_beta[i - 1] + beta[i]) % mod;
+    }
+    for (int i = 1; i <= n; i++) {
+        (ans += (((n + 1) * pre_alpha[i]) % mod) * pre_beta[i]) %= mod;
+        sum_alpha = (sum_alpha + pre_alpha[i]) % mod;
+        sum_beta = (sum_beta + pre_beta[i]) % mod;
+    }
+    long long tot = (sum_alpha * sum_beta) % mod;
+    ans = (ans - tot + mod) % mod;
+    cout << ans;
+}
\ No newline at end of file