A learned permutations of formula
#include <iostream> #include <string.h> #include <stdio.h> using namespace std; typedef long long LL; const LL p = 1e9 + 7; LL n,m; LL quick_mod(LL a, LL b) { LL ans = 1; a %= p; while(b) { if(b & 1) { ans = ans * a % p; b--; } b >>= 1; a = a * a % p; } return ans; } LL C(LL n, LL m) { if(m > n) return 0; LL ans = 1; for(int i=1; i<=m; i++) { LL a = (n + i - m) % p; LL b = i % p; ans = ans * (a * quick_mod(b, p-2) % p) % p; } return ans; } LL Lucas(LL n, LL m) { if(m == 0) return 1; return C(n % p, m % p) * Lucas(n / p, m / p) % p; } int main(){ int n,m,k; cin>>n>>k; if(k==0){ cout<<1<<endl; return 0; } LL ans=0; if(k%2 == 0){ m = k/2; ans = C(n-k+m-1,m-1); } else{ m = (k-1)/2; ans = C(n-k+m,m); } cout<<ans<<endl; return 0; }