#include <bits/stdc++.h>

using namespace std;

ifstream ci ("aria.in");
ofstream co ("aria.out");

long double a[100001], b[100001];

int main() {
    ios_base::sync_with_stdio(0);
    int n;
    long double sum=0;
    ci >> n;
    for (int i=1; i<=n; i++)
        ci >> a[i] >> b[i];
    for (int i=1; i<n; i++)
        sum+=(a[i+1]-a[i])*(b[i]+b[i+1]);
    sum+=(a[1]-a[n])*(b[n]+b[1]);
    sum/=2;
    if (sum<0)
        co << setprecision(6) << fixed << -sum;
    else
        co << setprecision(6) << fixed << sum;
    return 0;

}