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#include <bits/stdc++.h>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;
struct
{
long double x,y;
}v[100005];
long double rez;
int main()
{
fin>>n;
for (int i=1;i<=n;i++)
{
fin>>v[i].x>>v[i].y;
}
v[n+1]=v[1];
for (int i=1;i<=n;i++)
{
rez+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
}
fout<<abs(rez/2);
return 0;
}