#include<fstream>
#include<iostream>
using namespace std;
struct puncte{double x,y;
};
int main()
{ifstream fin("aria.in");
ofstream fout("aria.out");
    int n,i;
double aria=0;
puncte v[100005];
fin>>n;
for(i=1;i<=n;i++)
fin>>v[i].x>>v[i].y;
for(i=1;i<=n;i++)
    aria+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
   aria+=v[n].x*v[1].y-v[1].x*v[n].y;
if(aria<0.0)
    fout<<(double)((-1)*1/2*aria);
else
    fout<<(double)(1/2*aria);
    fout.close();
    return 0;
}