May be someone need it.

#include<iostream>
#include<math.h>
using namespace std;
int cnk(int k,int n)
{
    double res=1;
    for(int i=1;i<=k;++i)
        res=res*(n-k+i)/i;
    return res;
}
int hnk(int k,int n)
{
    return cnk(k,n+k-1);
}
int main()
{
    int n;
    cin>>n;
    int n_2=n/2,sum,n_9=n_2*9,e,res=0;
    for(int i=0;i<=n_9;++i)
    {
        sum=0;
        e=1;
        for(int j=1;j<=i/10;++j)
        {
            sum+=e*cnk(j,n_2)*hnk(i-j*10,n_2);
            e*=-1;
        }
        res+=pow(hnk(i,n_2)-sum,2.);
    }
    cout<<res;
    return 0;
}

//cnk(k,n)=n!/k!/(n-k)!

Edited by author 28.04.2012 22:48

can you please explain the algorithm of your code ?

dynamic solution

#include <iostream>
using namespace std;
int n, dp[10][85], sum;
int main()
{
    cin >> n;
    for(int i = 0; i < 10; i++)
        dp[1][i] = 1;
    for(int i = 2; i <= n / 2; i++)
        for(int j = 0; j <= 81; j++){
            for(int k = 0; k < 10; k++)
                if(k <= j)
                    dp[i][j] += dp[i - 1][j - k];
                if(i == n / 2)
                    sum += dp[i][j] * dp[i][j];
        }
    cout << max(sum, 10);
}

Please, can you explain your dp solution, or give hint. I can't understand it .

Please use meaningful function