#include <cstdio>

#define LL long long

struct Punct{
    int x,y;
};

const int N_MAX = 100000;

LL mod(LL x){
    if(x>0) return x;
    return -x;
}

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

    int n;
    Punct puncte[N_MAX];

    scanf("%d",&n);
    for(int i=0;i<n;++i){
        scanf("%d %d", puncte[i].x,puncte[i].y);
    }
    LL arie;
    arie=0l;
    for(int i=0;i<n-1;++i){
        arie+=(puncte[i].y+puncte[i+1].y)/2*(puncte[i+1].x-puncte[i].x);
    }

    printf("%lld", mod(arie));
    return 0;
}