#include <iostream>
#include <fstream>
#include <iomanip>

const int MAXN = 100000 + 5;

using namespace std;

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

int n;
long double x[MAXN],y[MAXN];

int main()
{
    in>>n;
    long double suma = 0;
    for(int i = 1; i <= n; i++)
        in>>x[i]>>y[i];
    x[n + 1] = x[1];
    y[n + 1] = y[1];
    for(int i = 1; i <= n; i++)
        suma += x[i] * y[i + 1] - x[i + 1] * y[i];
    out<<setprecision(5)<<fixed<<suma / 2;
    return 0;
}