#include <iostream>
#include <cstdio>
#include <cstring>
#include <vector>
#include <deque>
#include <queue>
#include <array>
#include <set>
#include <map>
#include <cmath>
#include <complex>
#include <algorithm>
#include <numeric>
#include <utility>
#include <tuple>
#include <bitset>
#include <cstdint>
#include <cassert>
#include <random>
#include <iterator>

using namespace std;
using i64 = int64_t;
using i32 = int32_t;

int n;
vector<int> x, y;
int main() {
    cin >> n;
    x.resize(n);
    y.resize(n);
    for (int i = 0; i < n; ++i) {
        cin >> x[i] >> y[i];
    }
    int a = 0;
    for (int i = 0; i < n; ++i) {
        int j = (i + 1) % n;
        a += x[i] * y[j] - x[j] * y[i];
    }
    printf("%.1f\n", abs(double(a)) / 2);
    return 0;
}
