#include <stdio.h>

int main(){
	FILE * file;
	file = fopen( "aria.in" , "r");
	int N;
	double sum = 0;
	fscanf(file,"%d",&N);
	printf("%d\n",N);
	double v[2*N+2];
	for(int i = 0 ; i < 2*N;i+=2){
		fscanf(file,"%lf %lf",&v[i],&v[i+1]);
		printf("%lf %lf \n",v[i],v[i+1]);
	}
	v[2 * N] = v[0];
        v[2 * N + 1] = v[1];
	for (int i = 0; i < 2*N; i += 2) {
            sum += v[i] * v[i + 3] - v[i + 1] * v[i + 2];
        }
	fclose(file);
	file = fopen( "aria.out" , "w");
	fprintf(file,"%.5f\n",sum/2);
	fclose(file);
	return 0;
}