#include <fstream>
#define det(i,j) a[i]*b[j] - b[i]*a[j]

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

const int nmax = 100005;
double a[nmax], b[nmax], sol;
int n, i;

int main() {
    f >> n;
    for (i = 1; i <= n; i++) {
        f >> a[i] >> b[i];
        if (i > 1) sol += det(i-1,i);
    }
    sol += det(n,1);
    if (sol<0) sol = -sol;
    g << sol/2;
    return 0;
}