http://projecteuler.net/problem=28 [Answer:669171001]

 

/*
43 44 45 46 47 48 49
42 21 22 23 24 25 26
41 20  7  8  9 10 27
40 19  6  1  2 11 28
39 18  5  4  3 12 29
38 17 16 15 14 13 30
37 36 35 34 33 32 31
right-up   1, 9, 25, 49 -> n*n
left-up    1, 7, 21, 43 -> n*n-n+1
left-down  1, 5, 17, 37 -> n*n-2*n+2
right-down 1, 3, 13, 31 -> n*n-3*n+3
sum        4, 24,76, 160 -> 4*n*n-6*n+6
n=2*k-1
4*n*n-12*n+12=16kk-16k+4-12k+6+6=16*k*k-28*k+16
sum=8*k*(k+1)*(2*k+1)/3-14*k*(k+1)+16*k-3 // -3 for three 1
*/
#include <iostream>

using namespace std;

int main()
{
    int n = 1001;
    int k = (n + 1) / 2;
    int sum = 8*k*(k+1)*(2*k+1)/3 - 14*k*(k+1) + 16*k - 3;
    cout << sum << endl;
    return 0;
}