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#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct segment{
double x,y;
};
segment v[100005];
int n;
double s;
int main()
{
fin>>n;
for(int i=1;i<=n;i++)
fin>>v[i].x>>v[i].y;
for(int i=1;i<n;i++)
{s+=((v[i].x*v[i+1].y)-(v[i+1].x*v[i].y));
//cout<<s<<endl;
}
s+=((v[n].x*v[1].y)-(v[1].x*v[n].y));
if(s<0)
s*=(-1);
fout<<setprecision(6)<<fixed<<double(s/2);
return 0;
}