#include <cstdio>

using namespace std;

int x[100005], y[100005];

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

    int n;
    long long sum = 0;

    scanf("%d", &n);

    for(int i = 1; i <= n; ++ i) {
        scanf("%d%d", &x[i], &y[i]);
    }

    for(int i = 2; i <= n; ++ i) {
        sum += (1LL * x[i - 1] * y[i] - 1LL * x[i] * y[i - 1]) / 2;
    }
    sum += (1LL * x[n] * y[1] - 1LL * x[1] * y[n]) / 2;

    printf("%lld\n", sum / 2);

    return 0;
}