#include <fstream>
#include <cstdio>
#include <cmath>
#include <iomanip>
#define N 100010

using namespace std;

long double x[N], y[N], S;
int n, i;

int main()
{
    ifstream fi("aria.in");
    ofstream fo("aria.out");
    fi >> n;
    for(i = 1; i <= n; i++) fi >> x[i] >> y[i];
    x[++n] = x[1]; y[n] = y[1];
    for(i = 1; i < n; i++) S += x[i] * y[i+1] - y[i] * x[i+1];
    S = S/2;
    if(S < 0) S = -S;
    fo << setprecision(10) << fixed;
    fo << S;
    return 0;
}