diff --git a/BZOJ/2821/2821.cpp b/BZOJ/2821/2821.cpp
new file mode 100644
index 00000000..62add4f3
--- /dev/null
+++ b/BZOJ/2821/2821.cpp
@@ -0,0 +1,111 @@
+#include <iostream>
+#include <cmath>
+#include <cstring>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+constexpr int N = 1e5 + 5,
+              M = 1005;
+
+int n, c, m, a[N], cnt[M][N], ans[M][N], res;
+int t, b, st[N], ed[N], pos[N], h[N];
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n >> c >> m;
+
+    t = std::sqrt(n);
+
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i];
+    }
+
+    for (b = 1;; b++) {
+        st[b] = (b - 1) * t + 1;
+        ed[b] = std::min(n, b * t);
+
+        if (b * t >= n) break;
+    }
+
+    for (int i = 1; i <= b; i++) {
+        for (int j = st[i]; j <= ed[i]; j++) {
+            pos[j] = i;
+            cnt[i][a[j]]++;
+        }
+    }
+
+    for (int i = 1; i <= b; i++) {
+        for (int j = 0; j <= c; j++) {
+            cnt[i][j] += cnt[i - 1][j];
+        }
+    }
+
+    for (int i = 1; i <= b; i++) {
+        for (int j = i; j <= b; j++) {
+            ans[i][j] = ans[i][j - 1];
+
+            for (int k = st[j]; k <= ed[j]; k++) {
+                h[a[k]]++;
+
+                if (h[a[k]] % 2 == 0) ans[i][j]++;
+                else if (h[a[k]] >= 3) ans[i][j]--;
+            }
+        }
+
+        memset(h, 0x00, sizeof(h));
+    }
+
+    while (m--) {
+        int l, r;
+
+        cin >> l >> r;
+
+        l = (l + res) % n + 1;
+        r = (r + res) % n + 1;
+        if (l > r) std::swap(l, r);
+
+        if (pos[l] == pos[r]) {
+            res = 0;
+
+            for (int i = l; i <= r; i++) {
+                h[a[i]]++;
+                if (h[a[i]] % 2 == 0) res++;
+                else if (h[a[i]] >= 3) res--;
+            }
+
+            for (int i = l; i <= r; i++) h[a[i]]--;
+
+        } else {
+            res = ans[pos[l] + 1][pos[r] - 1];
+
+            for (int i = l; i <= ed[pos[l]]; i++) {
+                h[a[i]]++;
+
+                int x = cnt[pos[r] - 1][a[i]] - cnt[pos[l]][a[i]];
+
+                if ((h[a[i]] + x) % 2 == 0) res++;
+                else if (h[a[i]] + x >= 3) res--;
+            }
+
+            for (int i = st[pos[r]]; i <= r; i++) {
+                h[a[i]]++;
+
+                int x = cnt[pos[r] - 1][a[i]] - cnt[pos[l]][a[i]];
+
+                if ((h[a[i]] + x) % 2 == 0) res++;
+                else if (h[a[i]] + x >= 3) res--;
+            }
+
+            for (int i = l; i <= ed[pos[l]]; i++) h[a[i]]--;
+            for (int i = st[pos[r]]; i <= r; i++) h[a[i]]--;
+        }
+
+        cout << res << endl;
+    }
+
+    return 0;
+}
diff --git a/BZOJ/2821/data/1.in b/BZOJ/2821/data/1.in
new file mode 100644
index 00000000..b950a1cc
--- /dev/null
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diff --git a/BZOJ/2821/data/1.out b/BZOJ/2821/data/1.out
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diff --git a/BZOJ/2821/data/2.in b/BZOJ/2821/data/2.in
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new file mode 100644
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