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ONLINE JUDGE/QBOJ/quanbai.cpp

@@ -0,0 +1,34 @@
+#include <bits/stdc++.h>
+#include <ext/pb_ds/assoc_container.hpp>
+#include <ext/pb_ds/tree_policy.hpp>
+
+using namespace std;
+using namespace __gnu_pbds;
+
+#define endl '\n'
+#define int long long
+#define inp freopen("file.inp", "r", stdin)
+#define out freopen("file.out", "w", stdout)
+#define TIME 1.0*clock()/CLOCKS_PER_SEC
+#define fastIO ios_base::sync_with_stdio(false); cin.tie(nullptr)
+template<typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
+template<typename K, typename V> using ordered_map = tree<K, V, less<K>, rb_tree_tag, tree_order_statistics_node_update>;
+
+const int MAXN = 1e6 + 5;
+const int MOD = 1e9 + 7;
+
+int n, sum;
+
+signed main() {
+    fastIO;
+    cin >> n;
+
+    for (int i = 0; i < n - 1; i++) {
+        int x;
+        cin >> x;
+        sum += x;
+    }
+
+    cout << ((n * (n + 1) >> 1) - sum);
+    return 0;
+}