Submission #188169

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Source Code Expand

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include<cassert>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include<bitset>

typedef long long ll;
#define REP(i,b,n) for(ll i=b;i<(ll)n;i++)
#define rep(i,n)   REP(i,0,n)
#define ALL(C)     (C).begin(),(C).end()
#define FOR(it,o)  for(__typeof((o).begin()) it=(o).begin(); it!=(o).end(); ++it)
#define dbg(x) cout << __LINE__ << ' ' << #x << " = " << (x) << endl



using namespace std;

//ハマったらチェックリスト見ろ!!

const int NN = 1005;

long double comb[NN][NN];

void makeComb(){
  rep(i, NN)rep(j, NN)comb[i][j] = 0;
  rep(i, NN){
    comb[i][0] = 1;
  }
  REP(i, 1, NN){
    REP(j, 1, NN){
      comb[i][j] = (comb[i-1][j-1] + comb[i-1][j]);
    }
  }
}

int main(){
    makeComb();
    ll N, D;
    cin >> N >> D;
    ll X, Y; 
    cin >> X >> Y;
    if(X % D || Y % D){
        cout << 0 << endl;
        return 0;
    }
    X /= D;
    Y /= D;
    long double cnt = 0;
    rep(a, N+1){
        ll b= a - X;
        if(b < 0)continue;
        rep(c, N+1){
            ll d = c - Y;
            if(d <0 )continue;
            if(a + b + c+d == N){
                cnt += comb[N][a] * comb[N-a][b] *comb[N-a-b][c] * comb[N-a-b-c][d];
            }
        }
    }
    rep(i, N){
        cnt /= 4;
    }
    printf("%.9Lf\n", cnt);

  return 0;
}

Submission Info

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