#pragma GCC optimize("Ofast") #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; typedef long long int ll; typedef unsigned long long ull; mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count()); ll myRand(ll B) { return (ull)rng() % B; } inline double time() { return static_cast(chrono::duration_cast(chrono::steady_clock::now().time_since_epoch()).count()) * 1e-9; } constexpr ll mod = 1e9+7; ll mod_pow(ll a,ll b){ a%=mod; if(b==0)return 1; if(b==1)return a; ll res=mod_pow(a,b/2)%mod; res*=res; res%=mod; if(b%2)res*=a; return res%mod; } struct perm{ private: int sz; vector p,invp; public: perm(int n){ sz=n+1; p.resize(sz),invp.resize(sz); p[0]=1; for(int i=1;i<=sz-1;i++){ p[i]=p[i-1]*i%mod; } invp[sz-1]=mod_pow(p[sz-1],mod-2); for(int i=sz-2;i>=0;i--){ invp[i]=invp[i+1]*(i+1)%mod; } } ll comb(ll x,ll y){ if(x> n >> m; if(n == 1){ cout << 1 << endl; return 0; } ll res = 0; { // 1,2,…,n,n,…,1 int cur = 0; for(int i=0;;i++){ if(cur > m) break; if((m-cur)%2 == 0)res += p.comb(m, cur+(m-cur)/2); if(i%2 == 0){ cur += n*2-1; } else{ cur += 1; } } } { int cur = 1; for(int i=0;;i++){ if(cur > m) break; if((m-cur)%2 == 0)res += p.comb(m, cur+(m-cur)/2); if(i%2 == 0){ cur += n*2-1; } else{ cur += 1; } } } cout << res%mod << endl; }