#include <stdio.h>

int main(){
  FILE *in = fopen("aria.in", "r");
  int n, i;
  fscanf(in, "%d", &n);
  long double aria = 0, x1, x2, y1, y2, xi, yi;
  fscanf(in, "%Lf%Lf", &xi, &yi);
  x1 = xi;  y1 = yi;
  for(i = 0; i < n; i++){
    fscanf(in, "%Lf%Lf", &x2, &y2);
    aria += x1 * y2 - y1 * x2;
    y1 = y2;  x1 = x2;
  }
  aria += x1 * yi - y1 * xi;
  if(aria < 0)  aria = -aria;
  aria /= 2.0;
  FILE *out = fopen("aria.out", "w");
  fprintf(out, "%.5Lf", aria);
  fclose(in);
  fclose(out);
  return 0;
}