#include <iostream>
#include <cstdio>
#define NMAX 100000

using namespace std;

struct point {
  double x, y;
} p[NMAX + 5];

int n;
double aria;

double cp(point p1, point p2) {
  return p1.x * p2.y - p1.y * p2.x;
}

double modul(double x) {
  if(x >= 0)
    return x;
  return -x;
}

int main() {
  freopen("aria.in", "r", stdin);
  freopen("aria.out", "w", stdout);

  scanf("%d", &n);
  for(int i = 1; i <= n; i++)
    scanf("%lf %lf", &p[i].x, &p[i].y);
  for(int i = 1; i <= n; i++)
    aria += cp(p[i], p[i % n + 1]) / 2;
  printf("%.15f", aria);

  return 0;
}