#include <iostream>
#include <fstream>

using namespace std;
int x[100001],y[100001];
int main()
{ifstream fin("aria.in");
ofstream fout("aria.out");
int n,i,s;
fin>>n;
for(i=1;i<=n;i++)
    fin>>x[i]>>y[i];
for(i=1,s=0;i<n;i++)
    s+=(x[i]*y[i+1]-x[i+1]*y[i]);
s+=x[n]*y[1]-x[1]*y[n];
if(s<0)
    fout<<-s/2;
else
    fout<<s/2;
    return 0;
}