#include <stdio.h>

struct Point {
   double x, y;
};

const int NMAX = 100005;
int N;

Point points[NMAX];

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", &points[i].x, &points[i].y);
   }

   double aria = 0.0;
   points[N+1] = points[1];

   for (int i = 1; i <= N; ++i) {
      aria += (points[i].x * points[i+1].y - points[i+1].x * points[i].y);
   }

   printf("%.5lf\n", aria * 0.5);

   fclose(stdin);
   fclose(stdout);
   return 0;
}