#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n, i;
double x, y, x2, y2, sum;
int main()
{
    fin >> n >> x >> y;
    for(i = 2; i <= n; i++)
    {
        fin >> x2 >> y2;
        sum += (y + y2) * (x2 - x);
        x = x2;
        y = y2;
    }
    sum = abs(sum/2);
    fout << sum;
    return 0;
}