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;
}

//