100168 C

#include <cstdio>
#include <cmath>
#include <algorithm>

using namespace std;

typedef long long ll;

#define FILE_NAME "area"

const int N = 100100;

pair<int, int> a[N];

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

    int n;
    scanf("%d", &n);
    for (int i = 0; i < n; ++i)
        scanf("%d%d", &a[i].first, &a[i].second);
    a[n] = a[0];
    ll sum = 0;
    for (int i = 1; i <= n; ++i)
        sum += (a[i].first - a[i - 1].first) * (a[i].second + a[i - 1].second);
    if (sum < 0) sum = -sum;
    printf("%.12lf\n", 0.5 * sum);

    return 0;
}