DP 預處理
//====================================================================||
// Name : MIXING INVITATIONS.cpp ||
// Date : 2013/6/8 上午2:50:07 ||
// Author : GCA ||
// 6AE7EE02212D47DAD26C32C0FE829006 ||
//====================================================================||
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <climits>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <cctype>
#include <utility>
using namespace std;
#ifdef ONLINE\_JUDGE
#define ll "%lld"
#else
#define ll "%I64d"
#endif
typedef unsigned int uint;
typedef long long int Int;
#define Set(a,s) memset(a,s,sizeof(a))
#define Write(w) freopen(w,"w",stdout)
#define Read(r) freopen(r,"r",stdin)
#define Pln() printf("\\n")
#define I\_de(x,n)for(int i=0;i<n;i++)printf("%d ",x\[i\]);Pln()
#define De(x)printf(#x"%d\\n",x)
#define For(i,x)for(int i=0;i<x;i++)
#define CON(x,y) x##y
#define Pmz(dp,nx,ny)for(int hty=0;hty<ny;hty++){for(int htx=0;htx<nx;htx++){\\
printf("%d ",dp\[htx\]\[hty\]);}Pln();}
#define M 55
#define PII pair<int,int\>
#define PB push\_back
#define oo INT\_MAX
#define Set\_oo 0x3f
#define Is\_debug true
#define debug(...) if(Is\_debug)printf("DEBUG: "),printf(\_\_VA\_ARGS\_\_)
#define FOR(it,c) for(\_\_typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define eps 1e-6
bool xdy(double x,double y){return x>y+eps;}
bool xddy(double x,double y){return x>y-eps;}
bool xcy(double x,double y){return x<y-eps;}
bool xcdy(double x,double y){return x<y+eps;}
int min3(int x,int y,int z){
int tmp=min(x,y);
return min(tmp,z);
}
int max3(int x,int y,int z){
int tmp=max(x,y);
return max(tmp,z);
}
Int n,m;
Int dp\[26\];
Int c\[26\]\[26\];
void pre(){
dp\[0\]=1;
dp\[1\]=0;
for(int i=2;i<26;i++){
dp\[i\]=(i-1)\*(dp\[i-1\]+dp\[i-2\]);
}
Set(c,0);
for(int i=0;i<26;i++){
c\[i\]\[0\]=1;
}
for(int i=1;i<26;i++){
for(int j=1;j<26;j++){
c\[i\]\[j\]=c\[i-1\]\[j-1\]+c\[i-1\]\[j\];
}
}
}
int main() {
ios\_base::sync\_with\_stdio(0);
pre();
while(~scanf("%lld%lld",&n,&m)){
Int ans=0;
for(int i=0;i<=m;i++){
ans+=c\[n\]\[i\]\*dp\[n-i\];
}
printf("%lld\\n",ans);
}
}