#include <fstream>

using namespace std;
ifstream in ("aria.in");
ofstream out ("aria.out");
int v1[100002],v2[100002];

int main()
{
    int n;
    int i;
    int a=0;
    in>>n;
    for(i=1;i<=n;i++)
    in>>v1[i]>>v2[i];
    v1[n+1]=v1[1];
    v2[n+1]=v1[1];
    for(i=1;i<=n;i++)
    {
        a=a+(v1[i]*v2[i+1]-v1[i+1]*v2[i-1]);
    }
    out<<a;
    return 0;
}