#include <fstream>
#include <iomanip>

using namespace std;

long long x[100005], y[100005];

int main() {
    ifstream f("aria.in");
    ofstream g("aria.out");
    
    int n;
    f >> n;
    
    for (int i = 0; i < n; i++) {
        f >> x[i] >> y[i];
    }
    
    long long sum = 0;
    for (int i = 0; i < n; i++) {
      int j; 
      if (i == n - 1) j = 0;
      else j = i + 1;
        sum += x[i] * y[j] - x[j] * y[i];
    }
    
    double rez = sum / 2.0;
    if (rez < 0) rez = -rez;
    
    g << fixed << setprecision(6) << rez;
    
    return 0;
}