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#include <cstdio>
#include <iostream>
#include <cmath>
#include <vector>
#include <algorithm>

using namespace std;

double dist(double x1, double y1, double x2, double y2) {
  return sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
}

int main() {
  int n;
  scanf("%d", &n);

  while(n) {
    double x1, y1, x2, y2, A = 0.0;
    vector<pair<double, double> > V(n+1);

    for(int i=0; i<n; i++)
      cin >> V[i].first >> V[i].second;

    if(n < 3) {
      printf("0\n");
      scanf("%d", &n);
      continue;
    }

    V[n].first = V[0].first;
    V[n].second = V[0].second;
    for(int i=0; i<V.size()-1; i++) {
      x1 = V[i].first, y1 = V[i].second;
      x2 = V[i+1].first, y2 = V[i+1].second;
      A += (x1*y2-x2*y1);
    }

    A /= 2.0;
    if(A<0) A*=(-1);

    printf("%.0lf\n", A);
    scanf("%d", &n);
  }

  return 0;
}