#include <bits/stdc++.h>
#define inf 10000000
using namespace std;
ifstream f ("numarare.in");
ofstream g ("numarare.out");
const int nmax=2e5+3;
int sol,v[nmax],d[nmax],k,n,c,r,mr,st,dr,sum;
int main()
{
    ios::sync_with_stdio(false);
    f>>k;
    f>>v[++n];
    for(int i=2;i<=k;++i)
    {
        v[++n]=inf;
        f>>v[++n];
    }
    c=0;
    r=0;
    for(int i=2;i<=n;i+=2)
    {
        if(r>=i)
        {
            mr=2*c-i;
            d[i]=min(d[mr],n-i);
        }
        st=i-1;
        dr=i+1;
        sum=v[st]+v[dr];
        while(st>=1&&dr<=n&&v[st]+v[dr]==sum)
        {
            d[i]+=2;
            st-=2;
            dr+=2;
        }
        sol+=d[i]/2;
        if(dr-2>r)
        {
            r=dr-2;
            c=i;
        }
    }
    g<<sol;
    return 0;
}