#include <stdio.h>
#define MAXN 100000
int x[MAXN + 1], y[MAXN + 1];

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