#include<cstdio>
int i,k,ok,x[5],n,a[9002],nr;
int valid(int k)
{
    int i;
    for(i=1;i<=k-1;i++)
    {
        if(x[k]==x[i])return 0;
    }
    return 1;
}
int main()
{
    freopen("medie.in","r",stdin);
    freopen("medie.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%d",&a[i]);
    }
    k++;
    x[k]=0;
    while(k>1)
    {
        ok=0;
        while(ok==0&&k<=3&&x[k]<n)
        {
            x[k]++;
            if(valid(k))ok=1;
        }
        if(ok==1)
        {
            if(k==3)
            {
                if(a[x[1]]==(a[x[2]]+a[x[3]])/2)nr++;
            }
            else {k++;x[k]=0;}
        }
        else k--;
    }
    printf("%d",nr);
    return 0;
}