#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <vector>
#define MOD 1024
#define MAXN 9000

#define BUF_SIZE 16384
char buf[BUF_SIZE];
int pbuf=BUF_SIZE;
FILE*fi,*fo;
inline char nextch(){
    if(pbuf==BUF_SIZE){
        fread(buf, BUF_SIZE, 1, fi);
        pbuf=0;
    }
    return buf[pbuf++];
}
inline long long nextnum(){
    long long a=0;
    char c=nextch();
    while((c<'0' || c>'9') && c!='-')
        c=nextch();
    int semn=1;
    if(c=='-'){
        semn=-1;
        c=nextch();
    }
    while('0'<=c && c<='9'){
        a=a*10+c-'0';
        c=nextch();
    }
    return a*semn;
}
class Hash{
    public:
    long long k=0, next[MAXN+1], *lista;
    long long val[MAXN+1];
    long long d[MAXN+1];
    Hash() {
        lista = new long long[MOD];
        memset(lista, 0x00, MOD*sizeof(int));
    }
    inline void insert(long long element, long long frecventa){
        k++;
        val[k]=element;
        d[k]=frecventa;
        next[k]=lista[(1LL*element+1LL*MOD*MOD)%MOD];
        lista[(1LL*element+1LL*MOD*MOD)%MOD]=k;
    }
    inline void erase(long long element){
        long long p=lista[(1LL*element+1LL*MOD*MOD)%MOD];
        if(p==0)
            return ;
        if(val[p]==element){
            lista[(1LL*element+1LL*MOD*MOD)%MOD]=next[lista[(1LL*element+1LL*MOD*MOD)%MOD]];
            return ;
        }
        while(next[p]!=0 && val[next[p]]!=element)
            p=next[p];
        if(next[p]!=0){
            next[p]=next[next[p]];
        }
    }
    inline long long find(long long element){
        long long p=lista[(1LL*element+1LL*MOD*MOD)%MOD];
        while(p!=0 && val[p]!=element)
            p=next[p];
        if(p!=0)
            return p;
        return 0;
    }
} H;

int v[9000];
int main(){
    fi=fopen("medie.in","r");
    fo=fopen("medie.out","w");
    int n=nextnum();
    for(int i=0;i<n;i++){
        v[i]=nextnum();
        if(!H.find(v[i]))
            H.insert(v[i], 1);
        else
            H.d[H.find(v[i])]++;
    }
    int c=0;
    for(int i=0;i<n;i++)
        for(int j=i+1;j<n;j++)
            if((v[i]+v[j])%2==0 && H.find((v[i]+v[j])/2)){
                if(v[i]==v[j])
                    c+=H.d[H.find((v[i]+v[j])/2)]-2;
                else
                    c+=H.d[H.find((v[i]+v[j])/2)];
            }
    fprintf(fo,"%d", c);
    fclose(fi);
    fclose(fo);
    return 0;
}