#include <bits/stdc++.h>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
double v[100005];
double c[100005];
int main()
{
    int n;
    in >> n;
    for(int i = 1; i <= n; i++)
        in >> v[i] >> c[i];
    v[n + 1] = v[1];
    c[n + 1] = c[1];
    double s;
    for(int i = 1; i <= n; i++)
        s += v[i] * c[i + 1] - v[i + 1] * c[i];
    out << setprecision(5) << fixed << s / 2.0;
    return 0;
}