#include <bits/stdc++.h>
#define NMAX 100001

using namespace std;

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

struct point{
    double x, y;
};

int n;
point A, B, S;
double aria;

double det(point B, point C) {
    return (B.x * C.y - B.y * C.x) / (double) 2;
}

int main()
{
    fin >> n;
    fin >> A.x >> A.y;
    S = A;
    for(int i = 2; i <= n; i++) {
        fin >> B.x >> B.y;
        aria += det(A, B);
        A = B;
    }
    aria += det(A, S);
    fout << fixed;
    fout << setprecision(5) << aria << '\n';
    return 0;
}