#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;
    long 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 * y2 - x2 * y1;
        x1 = x2;
        y1 = y2;
    }
    s += x2*b - a*y2;
    s = fabs(s / 2.0);
    fout << fixed << setprecision(20) << s;
    return 0;
}