Index: lib/numeric/modArith.cpp
==================================================================
--- lib/numeric/modArith.cpp
+++ lib/numeric/modArith.cpp
@@ -2,20 +2,21 @@
 //-------------------------------------------------------------
 // Modulo Arithmetics
 //
 // Verified by
 //   - TCO10 R3 LV3
+//   - SRM 545 LV2
 //-------------------------------------------------------------
 
-static const int MODVAL = 1000000007; // must be prime for op/
+static const int MODVAL = 1000000007;
 struct mint
 {
 	int val;
 	mint():val(0){}
-	mint(int    x):val(x%MODVAL) {} // x>=0
-	mint(size_t x):val(x%MODVAL) {} // x>=0
-	mint(LL     x):val(x%MODVAL) {} // x>=0
+	mint(int    x):val(x%MODVAL) {}
+	mint(size_t x):val(x%MODVAL) {}
+	mint(LL     x):val(x%MODVAL) {}
 };
 mint& operator+=(mint& x, mint y) { return x = x.val+y.val; }
 mint& operator-=(mint& x, mint y) { return x = x.val-y.val+MODVAL; }
 mint& operator*=(mint& x, mint y) { return x = LL(x.val)*y.val; }
 mint POW(mint x, LL e) { mint v=1; for(;e;x*=x,e>>=1) if(e&1) v*=x; return v; }
@@ -26,10 +27,21 @@
 mint operator/(mint x, mint y) { return x/=y; }
 vector<mint> FAC_(1,1);
 mint FAC(LL n) { while( FAC_.size()<=n ) FAC_.push_back( FAC_.back()*FAC_.size() ); return FAC_[n]; }
 mint C(LL n, LL k) { return k<0 || n<k ? 0 : FAC(n) / (FAC(k) * FAC(n-k)); }
 
+// Pascal's triangle: if O(1) nCk is needed.
+vector< vector<mint> > CP_(2001);
+mint C(LL n, LL k) {
+	if(CP_[0].empty()) {
+		CP_[0].push_back(1);
+		for(int nn=1; nn<CP_.size(); ++nn)
+		for(int kk=0; kk<=nn; ++kk)
+			CP_[nn].push_back( (kk?CP_[nn-1][kk-1]:0) + (kk<nn?CP_[nn-1][kk]:0) );
+	}
+	return k<0 || n<k ? 0 : CP_[n][k];
+}
 
 
 
 /*
 // MODVAL must be a prime!!