#include<bits/stdc++.h>

using namespace std;

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

const int NMax = 1e5 + 1;

int n;
long double x[NMax], y[NMax];

int main()
{
    f>>n;

    for(int i = 1; i <= n; i++)
        f>>x[i]>>y[i];

    x[n+1] = x[1];
    y[n+1] = y[1];

    int s = 0;
    for(int i = 1; i <= n; i++)
    {
        s += x[i] * y[i + 1] - x[i+1] * y[i];
    }

    g<<fixed << setprecision(6) <<(long double)fabs(s/2.0f);

    return 0;
}