#include <iostream>
#include <vector>
using namespace std;
int main(void){
    int N,ans=0;
    cin >> N;
    vector<int> X(N),Y(N);
    for(int i=0;i<N;i++)cin >> X[i] >> Y[i];
    for(int i=0;i<N;i++){
        ans+=(X[i]*Y[(i+1)%N]-X[(i+1)%N]*Y[i]);
    }
    cout << ans << endl;
}