#include<fstream>

using namespace std;

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

int xuri[100001], yuri[100001];

int main(){
    int n;
    fin>>n;
    for(int i=1; i<=n; i++){
        fin>>xuri[i]>>yuri[i];
    }
    int s=0;
    for(int i=1; i<n; i++){
        s+=xuri[i]*yuri[i+1]-xuri[i+1]*yuri[i];
    }
    s+=xuri[n]*yuri[1]-xuri[1]*yuri[n];
    s/=2;
    fout<<s;
    return 0;
}