#include <bits/stdc++.h>
 
using namespace std;
 
inline void Open(const string Name) {
    #ifndef ONLINE_JUDGE
        (void)!freopen((Name + ".in").c_str(), "r", stdin);
        (void)!freopen((Name + ".out").c_str(), "w", stdout);
    #endif
}

struct point{
    long double x, y;
} v[100001];
 
long double area;

int N;
 
inline void computeArea() {
    int j = N;
    for(int i = 1;i <= N;i++)  {
        area += (v[j].x + v[i].x) * (v[j].y - v[i].y);
        j = i;
    }
}
 
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
 
    Open("aria");
 
    cin >> N;
    for(int i = 1;i <= N;i++)
        cin >> v[i].x >> v[i].y;
 
    computeArea();

    cout << fixed << setprecision(5) << abs(area / 2.0);
 
    
    return 0;
}