#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int main()
{
    fin.precision(5);
    fout.precision(5);
    int n;
    float x0, y0, x, y, xN, yN, aria;
    fin>>n;
    fin>>x0>>y0;
    x = x0;
    y = y0;
    while(fin>>xN>>yN) {
        aria +=(x*yN-y*xN);
        x = xN;
        y = yN;
    }
    aria +=(xN*y0-yN*x0);
    fout<<aria/2;
    fin.close();
    fout.close();
    return 0;
}