/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2021.03.31 22:02:19
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;

int main() {
    int n;cin >> n;
    ll ans = 0;
    ll x1,y1,x2,y2;cin >> x1 >> y1 >> x2 >> y2;
    for (int i = 2; i < n; i++) {
        ll x,y;cin >> x >> y;
        ans += abs((x1-x)*(y2-y)-(x2-x)*(y1-y));
        x2 = x;
        y2 = y;
    }
    cout << ans << endl;
    return 0;
}