#include <fstream>
#include <bits/stdc++.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

int N;
double a,b;
double c,d;
double _aria = 0;

int main()
{
    fin>>N;
    fin>>a>>b;
    for (int i = 1; i <= N-2; i++) {
        fin>>c>>d;

        _aria += abs(a*d - c*b);

        a = c;
        b = d;
    }

    fout<<_aria;
    return 0;
}