#include <stdio.h>

template<class T>T abs(T v){ return v>0?v:-v; }

double area(int x1, int y1, int x2, int y2, int x3, int y3){
	return abs((x1*y2 + x2*y3 + x3*y1 - y2*x3 - y1*x2 - y3*x1)/2.0);
}
int main(){
    int n;
    
    scanf("%d",&n);
	double total = 0.0;
    for (int i=0;i<n;++i){
      	int x[4], y[4];
	for(int j=0;j<4;j++){
		scanf("%d %d", x+j, y+j);
	}
	double S = area(x[0], y[0], x[1], y[1], x[2], y[2]) + area(x[1], y[1], x[2], y[2], x[3], y[3]);
total += S;    }
	printf("%lf", total);
    return 0;
}