#include<fstream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
float x[100001],y[100001];
int n;

void citire()
{
    int i;
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
}

float aria()
{
    float S=0;
    int i;
    for(i=1;i<=n;i++)
        S=S+x[i]*y[i+1]-x[i+1]*y[i];
    S=S/2;
    return S;
}

int main()
{
    citire();
    fout<<aria();
}