#include <fstream>
#include <iomanip>

using namespace std;

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

int n, i;
double aria, x[100001], y[100001], d;

int main(){
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>x[i]>>y[i];
    if(n == 1 || n== 2){
        fout<<setprecision(6)<<fixed<<aria;
        return 0;
    }
    for(i=3;i<=n;i++)
        d += (x[i-1] - x[1])*(y[i] - y[1]) - (x[i] - x[1])*(y[i-1] - y[1]);
    d = max(d, -d);
    fout<<setprecision(7)<<fixed<<d/2;
    return 0;
}