#include <cstdio>
#include <vector>

using namespace std;

typedef long long ll;

ll det(pair<int, int> a, pair<int, int> b)
{
    return 1ll*a.first*b.second - 1ll*a.second*b.first;
}
int n;
ll aria;
vector<pair<int, int> >v;

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int x, y;
    scanf("%d", &n);
    for(int i=1; i<=n; i++)
    {
        scanf("%d%d", &x, &y);
        v.push_back({x, y});
    }
    v.push_back(v[0]);
    vector<pair<int, int> >::iterator it;
    for(int i=0; i<v.size()-1; i++)
        aria += det(v[i], v[i+1]);
    aria /= 2;
    printf("%lld\n", aria);
}