#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

const int NMax = 100005;
struct node{
    long double x,y;
}A,B,C;

int main()
{
    int n;
    fin >> n >> A.x >> A.y;
    C = A;
    long double ans = 0;
    for(int i = 2; i <= n; i++){
        fin >> B.x >> B.y;
        ans = ans + ((A.x * B.y) - (B.x * A.y));
        A = B;
    }
    ans = ans + ((A.x * C.y) - (C.x * A.y));
    ans /= 2;
    fout << fixed << setprecision(6) << ans;
    return 0;
}