#include <bits/stdc++.h>
#define x first
#define y second

using namespace std;

int getsquere(pair<int, int> p1, pair<int,int> p2){
        return (p1.y + p2.y) * (p2.x - p1.x);
}
int main() {
    int N;
    cin >> N;
    pair<int, int> coords[N];
    for (int i=0;i<N;i++){
        cin >> coords[i].x;
        cin >> coords[i].y;
    }
    int s = 0;
    for (int i=0;i<N;i++){
        if (i!=0){
            s += getsquere(coords[i-1], coords[i]);
        }else{
            s += getsquere(coords[N-1], coords[0]);
        }
    }
    printf("%.1f", fabs(((double)s)/2));
    return 0;
}