#include <iostream>
using namespace std;


int main(){
    long int num;
    cin >> num;
    long long int point[num][2];
    for (int i = 0; i < num; i++){
        cin>>point[i][0]>>point[i][1];
    }
    long long int a=0;
    for (int i = 0; i < num-1; i++){
        a+=point[i][0]*point[i+1][1]-point[i+1][0]*point[i][1];
    }
    a+=point[num-1][0]*point[0][1]-point[0][0]*point[num-1][1];
    long long int res = abs(a)*0.5;
    cout << res;

    return 0;
}