#include <bits/stdc++.h>

using namespace std;
vector<double> X, Y;

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    int N;
    scanf("%d", &N);
    X.resize(N + 1);
    Y.resize(N + 1);

    for(int i = 1; i <= N; ++i)
        scanf("%lf%lf", &X[i], &Y[i]);
    X[0] = X[N];
    Y[0] = Y[N];
    double aria = 0;
    for(int i = 0; i < N; ++i)
        aria += (X[i] - X[i + 1])*(Y[i + 1] + Y[i]);

    printf("%.8f\n", aria / 2);

    return 0;
}