This (probably) sucks.

decimal-spiral.c

#include <stdio.h>

// Should run in O(1) space (ignoring output) and O(n^3) time, assuming TCO.

int solve(int n, int x, int y) {
    int k = (n - 1) * (n + 3) / 2;

    if (y == 0) {
        // First Row
        return k - x;
    } else if (y == 1) {
        // Second Row
        if (x == n - 1) {
            return k - x - 1;
        } else {
            return -1;
        }
    } else if (y == n - 2) {
        // Second-Last Row
        if (x == 0) {
            return k - 3*(n - 1) - 1;
        } else if (x == n - 1) {
            return k - 2*(n - 1) + 1;
        } else {
            return -1;
        }
    } else if (y == n - 1) {
        // Last Row
        return k - 3*(n - 1) + x;
    } else {
        // Middle
        if (x == 0) {
            return k - 4*(n - 1) + y;
        } else if (x == 1) {
            if (y == 2) {
                return k - 4*(n - 1) + y - 1;
            } else {
                return -1;
            }
        } else if (x == n - 1) {
            return k - (n - 1) - y;
        } else if (x == n - 2) {
            return -1;
        } else {
            return solve(n - 4, x - 2, y - 2);
        }
    }

    return -1;
}

void main(void) {
    int n, k;
    printf("Enter size: ");
    scanf("%d", &n);

    for (int y = 0; y < n; y++) {
        for (int x = 0; x < n; x++) {
            k = solve(n, x, y);
            if (k == -1) printf("-");
            else printf("%d", k % 10);
        }
        printf("\n");
    }
}

may i ask how did u get k = (n-1)(n+3)/2 ?