#include <bits/stdc++.h>
using namespace std;
const int nmx = 1e5 + 3;
typedef valarray<float> P;
#define cin fin
#define cout fout
ifstream fin("aria.in");
ofstream fout("aria.out");
P p[nmx];
float aria(P a, P b, P c){
    P ab = b-a, ac = c-a;
    return .5*abs(ab[0]*ac[1]-ab[1]*ac[0]);
}
int main(){
    int n;
    cin >> n;
    for(int i=0;i<n;i++){
        p[i] = {0,0};
        cin >> p[i][0] >> p[i][1];
    }
    float arie = 0;
    for(int i=1;i<n-1;i++)
        arie += aria(p[0],p[i],p[i+1]);
    cout << fixed << arie;
}