#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
using namespace std;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int n; cin >> n;
    int ans = 0;
    int x, y; cin >> x >> y;

    vector<int> xp(n), yp(n);
    for(int i=0; i<n-1; ++i) cin >> xp[i] >> yp[i];
    for(int i=0; i<n-2; ++i){
        int tx, ty, sx, sy;
        sx = xp[i]-x;
        sy = yp[i]-y;
        tx = xp[i+1]-x;
        ty = yp[i+1]-y;
        ans += abs(sx*ty - sy*tx);
    }
    cout << ans << endl;
}