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#include <fstream>
#include <iostream>
#include <vector>
#include <iomanip>
using namespace std;
ifstream be("aria.in");
ofstream ki("aria.out");
int main()
{
int n;
be>>n;
vector<pair<long double,long double> >a(n+1);
for(int i=0;i<n;i++){
be>>a[i].first>>a[i].second;
}
a[n].first=a[0].first,a[n].second=a[0].second;
long double sum=0;
for(int i=0;i<n;i++)
sum+=((a[i].first*a[i+1].second)-(a[i+1].first*a[i].second));
if(sum<0)
sum=-sum;
sum/=2.0;
ki<<setprecision(10)<<sum;
return 0;
}