euler28

Starting with the number 1 and moving to the right in a clockwise direction a 5 by 5 spiral is formed as follows:

21 22 23 24 25
20  7  8  9 10
19  6  1  2 11
18  5  4  3 12
17 16 15 14 13

It can be verified that the sum of the numbers on the diagonals is 101.

What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed in the same way?
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#include <stdio.h>

#define N 1001/2


int main(int argc, const char *argv[])
{
    int    sum = 1;
    int i;

    for (i = 1; i <= N; i++)
        sum += 4*(2*i+1)*(2*i+1) - 12*i;

    printf("sum: %d\n", sum);

    return 0;
}

注意N在某值时可能导致溢出。