#include <fstream>
using namespace std;
int x[100010],y[100010];
long long aria;
int main()
{
    ifstream fi("aria.in");
    ofstream fo("aria.out");
    int n,i;
    fi>>n;
    for(i=1;i<=n;++i)
        fi>>x[i]>>y[i];
    for(i=1;i<n;i++)
        aria+=(x[i]*y[i+1]-x[i+1]*y[i]);
    aria+=(x[n]*y[1]-x[1]*y[n]);
    fo<<aria/2;
    return 0;
}