#include <fstream>
#include <iomanip>
#define DIM 100010
using namespace std;
ifstream fin ("aria.in");
ofstream fout("aria.out");
pair<int, int> v[DIM];
int sol;
int n, i;
int aria(pair<int, int> a, pair<int, int> b, pair<int, int> c) {
return (b.first-a.first) * (c.second-a.second) -
(c.first-a.first) * (b.second-a.second);
}
int main() {
fin>>n;
for (i=1;i<=n;i++) {
fin>>v[i].first>>v[i].second;
}
v[0] = v[n];
for (i=0;i<n;i++) {
sol += aria(v[0], v[i], v[i+1]);
}
fout<<setprecision(0)<<fixed<<sol/2.0;
}