#include <stdio.h>
#define MAX 100
#include <stdlib.h>
double TriangleArea(int n,double a[MAX][MAX]);
int main()
{
    int i,j;
    int n=4;
    double a[MAX][MAX];
    for(i=0;i<n;i++)
    {
        for(j=0;j<n;j++)
        {
            printf("please input the num\n");
            scanf("%lf",&a[i][j]);
        }
    }
    printf("%lf",TriangleArea(n,a));
    return 0;
}
double TriangleArea(int n,double a[MAX][MAX])
{
    int p,i,j,k,x;
    double sum=0;
    double b[MAX][MAX]={{0}};//atention
    if(n == 1)
            return a[0][0];
    for(i=0;i<n;i++)//控制展开元素
    {
        for(j=0;j<n-1;j++)
        {
            for(k=0;k<n-1;k++)
            {
                if(j<i)
                    p=0;
                else
                    p=1;
                b[j][k]=a[j+p][k++];         
            }
        }
        if(i%2==0)
            x=1;
        else
            x=(-1);   
        //printf("%lf",a[i][1]);   
        sum+=a[i][0] * x * TriangleArea(n-1,b);
        //printf("sum=%lf\n",sum);        
    }
    return sum;
}