#include <fstream>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
double aria;
struct Punct
{
    double x, y;
} v[210];

int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
        for(int i=1;i<n;++i)
            aria+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
    aria+=(v[n].x*v[1].y-v[1].x*v[n].y)/2;
    g<<aria;
    return 0;
}