求任意多边形面积--改革春风吹满地

#include<stdio.h>
int main( )
{
int n;
int x[3];
int y[3];
double s;
while( scanf("%d",&n) == 1 )
{
if ( n == 0 )
{
break;
}
if( n >= 3 && n <= 100 )
{
s = 0;
scanf("%d %d",&x[0],&y[0]);
x[2] = x[0];
y[2] = y[0];
while( --n )
{
scanf("%d %d",&x[1],&y[1]);
s += x[0]*y[1] - x[1]*y[0];
x[0] = x[1];
y[0] = y[1];
}
s += x[0]*y[2] - x[2]*y[0];
printf("%.1lf\n", s/2);
}
}
return 0;
}