#include <iostream>
#include <fstream>

using namespace std;

ifstream in("aria.in");
ofstream out("aria.out");

struct hatz{
    int x,y;
};

hatz v[100005];
int n;
long long suma;
int main()
{
    in>>n;
    for(int i=1;i<=n;++i)
        in>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(int i=1;i<=n;++i){
        suma+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
    }
    out<<suma;
    return 0;
}