#include <bits/stdc++.h>
#define ll long long
#define lli long long int
#define endl "\n"
using namespace std;

int main(){
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    int ans = 0;
    int n;
    int x[100002],y[100002];
    cin >> n;
    for(int i=1;i<=n;i++)
        cin >> x[i] >> y[i];
    x[n+1] = x[1];
    y[n+1] = y[1];
    for(int i=1;i<=n;i++)
        ans+=x[i]*y[i+1] - x[i+1] * y[i];
    cout << ans/2;
	return(0);
}