#include <iostream>
#include <fstream>
#include <algorithm>
#include <cmath>
#include <iomanip>

using namespace std;

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

int main(){
    int n,i;
    double a,b,x1,x2,y1,y2,s;
    fin >> n;
    fin >> a >> b;
    x1 = a;
    y1 = b;
    s = 0;
    for (i=2;i<=n;++i) {
        fin >> x2 >> y2;
        s += (x1 - x2) * (y1 + y2);
        x1 = x2;
        y1 = y2;
    }
    s = fabs(s / 2.0);
    fout << fixed << setprecision(5) << s;
    return 0;
}