#include<fstream>
#include<cmath>
using namespace std;
ifstream fin("copaci.in");
ofstream fout("copaci.out");
unsigned long long n,i,B,c,d,x,y,a,b;
long A;
int main()
{
fin>>n;fin>>c>>d;
for(i=2;i<=n;i++)
{
    fin>>a>>b;

    A+=c*b-a*d;

    if(a>c)
    x=a-c;
    else
        x=c-a;
    if(b>d)
    y=b-d;
    else
        y=d-b;

    if(x>y)
swap(x,y);
    while(y)
{
    c=x%y;
    x=y;
    y=c;
}B+=x;
B++;
    c=a;
    d=b;
}A=A/2;
B=B/2;B--;
fout<<A+1-B;
}