#include <iostream>
#include <fstream>
using namespace std;
int N;
long double aria=0;
struct poz
{
    int x;
    int y;
} v[100001];
ifstream fin ("aria.in");
ofstream fout ("aria.out");
int main()
{
fin>>N;
for (int i=1; i<=N; i++)
fin>>v[i].x>>v[i].y;
v[N+1]=v[1];
for(int i=1; i<=N; i++)
    aria+=(v[i].x)*(v[i+1].y)-(v[i+1].x)*(v[i].y);

fout<<aria/2;
}