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#include <fstream>
#define NMAX 100010
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
long long n,i;
double x[NMAX],y[NMAX];
double s=0;
int main()
{
in>>n;
for(i=1;i<=n;i++)
{
in>>x[i]>>y[i];
}
x[n+1]=x[1];
y[n+1]=y[1];
for(i=1;i<=n;i++)
{
s=s+x[i]*y[i+1]-y[i]*x[i+1];
}
out<<s/2;
in.close();
out.close();
return 0;
}