#include <bits/stdc++.h>

#define f first
#define s second

using namespace std;

ifstream F("aria.in");
ofstream G("aria.out");

pair<double, double> p[100005];
int n;
double S;

double det(pair<double, double> pi, pair<double, double> pi_1){
    return (pi.f*pi_1.s-pi.s*pi_1.f)/2;
}

int main()
{
    F >> n;
    for(int i = 1; i <= n; ++ i){
        F >> p[i].f >> p[i].s;
    }
    p[0] = p[n];
    for(int i = 1; i <= n; ++ i){
        S+=det(p[i-1], p[i]);
    }
    G << setprecision(5) << fixed << S;
    return 0;
}