#include <iostream>
#include <fstream>
using namespace std;
ifstream f("numarare.in");
ofstream g("numarare.out");
 int n,v[100005],sol[100005],res=0;
int main()
{ int i;
    f>>n;
   for(i=1;i<=n;i++)
    f>>v[i];
     sol[2]=1; sol[3]=1;
   for(i=4;i<=n;i++)
    {if (v[i-2]-v[i-3]==v[i]-v[i-1])
      sol[i]=sol[i-2]+1; else sol[i]=1;
      res+=sol[i];
    }
    g<<res+(n>=2)+(n>=3);
    return 0;
}