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#include <iostream>
#include <fstream>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int main()
{
double a=0,x[100],y[100];
int i,N;
in>>N;
for(i=1; i<=N; i++)
{
in>>x[i]>>y[i];
}
for(i=1; i<=N; i++)
{
a+=(x[i]*y[i+1]-x[i+1]*y[i]);
}
a+=x[N]*y[1]-x[1]*y[N];
out<<a/2;
return 0;
}