Submission #188118

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Source Code Expand

#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <climits>
#include <cfloat>
#include <functional>
using namespace std;

double combination(int n, int r)
{
    if(n < r)
        return 0;
    if(n-r < r)
        r = n-r;
    double ret = 1;
    for(int i=0; i<r; i++){
        ret *= (n--);
        ret /= i+1;
    }
    return ret;
}

int main()
{
    int n, d, x, y;
    cin >> n >> d >> x >> y;

    if(x % d != 0 || y % d != 0){
        cout << 0 << endl;
        return 0;
    }
    x = abs(x / d);
    y = abs(y / d);

    double ret = 0.0;
    for(int i=0; i<=n; ++i){
        if(i < x || n-i < y)
            continue;
        if((i - x) % 2 != 0)
            continue;
        if((n - i - y) % 2 != 0)
            continue;

        ret += combination(n,   i              ) / pow(2.0, n) *
               combination(i,   (i - x)     / 2) / pow(2.0, i) *
               combination(n-i, (n - i - y) / 2) / pow(2.0, n-i);
    }

    cout.setf(ios_base::fixed, ios_base::floatfield);
    cout << setprecision(10) << ret << endl;

    return 0;
}

Submission Info

Judge Result