determinant calculation

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int n;
int *p;
int *c;
double *m;
double det;

int inversions(int *p) {
    extern int n;
    int s = 0;

    for (int i = 0; i < n; i++)
        for (int j = i + 1; j < n; j++)
            s += *(p + i) > *(p + j);
    return s;
}

void swap(int i, int j) {
    extern int *c;

    int t = *(c + i);
    *(c + i) = *(c + j);
    *(c + j) = t;
}

void calc(int k) {
    extern int n;
    extern int *p;
    extern int *c;
    extern double *m;
    extern double det;

    if (k == n) {
        for (int i = 0; i < n; i++)
            *(p + i) = *(c + i);

        double a = 1.0;
        for (int i = 0; i < n; i++) {
             a *= *(m + (*(p + i) - 1) * n + i);
        }
        det += pow(-1, inversions(p)) * a;
    }
    else {
        for (int j = k; j < n; j++) {
             swap(k, j);
             calc(k + 1);
             swap(k, j);
        }
    }
}

int main(int argc, char *argv[]) {
    extern int n; // matrix size
    extern int *p; // rearrangements
    extern int *c; // canonical sequence
    extern double *m; // matrix
    extern double det;

    printf("РАЗМЕР МАТРИЦЫ = "); scanf("%d", &n);
    putchar('\n');

    p = (int *)malloc(sizeof(int) * n);
    c = (int *)malloc(sizeof(int) * n);
    m = (double *)malloc(sizeof(double) * n * n);
    det = 0.0;

    for (int i = 0; i < n; i++)
        *(c + i) = i + 1;

    for (int i = 0; i < n; i++) {
        printf("---СТРОКА #%d---\n", i + 1);
        for (int j = 0; j < n; j++) {
            printf("столбец #%d = ", j + 1);
            scanf("%lf", m + i * n + j);
        }
        putchar('\n');
    }
    putchar('\n');

    calc(0);

    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++)
            printf("%-g ", *(m + i * n + j));
        putchar('\n');
    }
    putchar('\n');
    putchar('\n');

    printf("ОПРЕДЕЛИТЕЛЬ = %g\n", det);

    return 0;
}