#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("YES");}else{puts("NO");}
#define MAX_N 200005

// 凸多角形の面積の2倍を返す
// 頂点は反時計回りにソートして渡す。
ll calcSurfaceArea(vector<int> x, vector<int> y){
    int n = x.size();
    x.push_back(x[0]);
    y.push_back(y[0]);
    ll res = 0;
    rep(i,n){
        res += (x[i] - x[i+1]) * (y[i] + y[i+1]);
    }
    return res;
}

int main() {
    int n;
    cin >> n;
    vector<int> x(n), y(n);
    rep(i,n) cin >> x[i] >> y[i];

    int ans = calcSurfaceArea(x,y);

    cout << ans << endl;
    return 0;
}