#include <fstream>
#include<iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
    long double x[100010], y[100010], s = 0;
    int n;
    fin>>n;
    for(int i = 0; i<n; ++i){
        fin>>x[i]>>y[i];
    }
    x[n] = x[0];
    y[n] = y[0];
    for(int i = 0; i<n; ++i){
        double term =  (x[i]*y[i+1] - x[i+1]*y[i]);
        s += term;
    }
    s/=2;
    fout << fixed << setprecision (7)<<s;
    return 0;
}