(UVA) 11282 – Mixing Invitations

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#include<bits/stdc++.h>


#define FOR(i, s, e) for(int i=s; i<e; i++)
#define loop(i, n) FOR(i, 0, n)
#define sf scanf
#define pf printf
#define pb push_back
#define MP make_pair
#define fr first
#define sc second
#define ll long long
#define dd double
#define all(v) v.begin(), v.end()
#define PI acos(-1.0)
#define mem(ara, value) memset(ara, value, sizeof(ara))
#define paii pair<int, int>
#define pall pair<ll, ll>
#define SZ(a) int(a.size())
#define read(nm) freopen(nm, "r", stdin)
#define write(nm) freopen(nm, "w", stdout)

#define take(args...) asdf,args
#define dump(x) cerr<<#x<<" = "<<x<<endl
#define debug(args...) cerr,args; cerr<<endl;
using namespace std;


template<typename T>
ostream& operator<<(ostream& output, vector<T>&v)
{
    output<<"[ ";
    if(SZ(v))
    {
        output<<v[0];
    }
    FOR(i, 1, SZ(v))
    {
        output<<", "<<v[i];
    }
    output<<" ]";
    return output;
}

template<typename T1, typename T2>
ostream& operator<<(ostream& output, pair<T1, T2>&p)
{
    output<<"( "<<p.fr<<", "<<p.sc<<" )";
    return output;
}




template<typename T>
ostream& operator,(ostream& output, T x)
{
    output<<x<<" ";
    return output;
}



struct ASDF{
    ASDF& operator,(int &a) {
        sf("%d", &a);
        return *this;
    }
    ASDF& operator,(long int &a){
        sf("%ld", &a);
        return *this;
    }
    ASDF& operator,(long long int &a){
        sf("%lld", &a);
        return *this;
    }
    ASDF& operator,(char &c){
        sf("%c", &c);
        return *this;
    }
    ASDF& operator,(double &d){
        sf("%lf", &d);
        return *this;
    }

    template<typename T>
    ASDF& operator,(T &a){
        cin>>a;
        return *this;
    }
}asdf;



//Header ends here

#define MAXX 27

ll D[MAXX];

ll nCr[MAXX][MAXX];

void init()
{

    /// pre calculate dearrangement
    D[0] = 1;
    D[1] = 0;
    for(int i=2; i<MAXX; i++)
    {
        D[i] = (i-1)*(D[i-1] + D[i-2]);
    }



    ///

    for(int i=0; i<MAXX; i++)
    {
        nCr[i][0] = 1;
    }

    for(int i=1; i<MAXX; i++)
    {
        for(int j=1; j<=i; j++)
        {
            nCr[i][j] = nCr[i-1][j] + nCr[i-1][j-1];
        }
    }

    //cout<<nCr[4][2];

}






int main()
{
    init();

    int n, m;

    while(sf("%d %d", &n, &m) == 2)
    {
        ll cnt = 0;

        for(int i=0; i<=m; i++)
        {
            cnt += nCr[n][i] * D[n-i];
        }
        cout<<cnt<<endl;
    }




}