#include <stdio.h>
#define NMax 100001
FILE *fin = fopen("aria.in", "r");
FILE *fout = fopen("aria.out", "w");

int n;
long double x[NMax], y[NMax];
long double areaTri(int a, int b, int c) {
	long double rez = 0;
	rez += x[a] * y[b];
	rez += y[a] * x[c];
	rez += x[b] * y[c];

	rez -= x[a] * y[c];
	rez -= y[a] * x[b];
	rez -= y[b] * x[c];

	return rez;
}
int main() {

	fscanf(fin, "%d", &n);

	for (int i = 0; i < n; i++) {
		fscanf(fin, "%lf %lf", &x[i], &y[i]);
	}
	long double area = 0;
	for (int i = 1; i + 1 < n; i++) {
		area += areaTri(0, i, i + 1);
	}
	if (area < 0) {
		area = -area;
	}
	fprintf(fout, "%.6lf\n", area / 2.0);

	fclose(fin);
	fclose(fout);
	return 0;
}