HDOJ 2036 多边形的面积

整数坐标下，面积的2倍也是整数；

多边形面积公式；

# include <stdio.h>



# define N 105



int n;

int x[N], y[N];



double area(void)

{

    int i;

    double ret;

    

    ret = 0;

    for (i = 0; i < n-1; ++i)

    {

        ret += x[i]*y[i+1] - x[i+1]*y[i];

    }

    ret += x[n-1]*y[0] - x[0]*y[n-1];

    

    return ret*0.5;

}



int main()

{

    int i;

    

    while (~scanf("%d", &n))

    {

        if (n == 0) break;

        for (i = 0; i < n; ++i) scanf("%d%d", &x[i], &y[i]);

        printf("%.1lf\n", area());

    }

    

    return 0;

}

//