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source/Math/FloorSum.cpp

@@ -0,0 +1,7 @@
+ll floor_sum(ll a, ll b, ll c, ll n)
+{
+    if(a == 0) return b/c*n;
+    if(a>=c or b>=c) return n*(n-1)/2 * (a/c) + n * (b/c) + floor_sum(a%c, b%c, c, n);
+    ll m = (a*(n-1)+b)/c;
+    return m*n - floor_sum(c, c-b+a-1, a, m);
+}