#include <cstdio>
int f[70005],fr[70005],r;
int main () {
freopen("medie.in","r",stdin);
freopen("medie.out","w",stdout);
int i,j,n,a;
scanf("%d",&n);
for(i=1;i<=n;++i){
scanf("%d",&f[i]);
}
for(i=1;i<=n;++i){
fr[f[i]]++;
}      
for(i=2;i<=n;++i){
for(j=1;j<=n;++j){
if(i>j&&(f[i]+f[j])%2==0){
a=(f[i]+f[j])/2;
r=r+fr[a];
if(f[i]==f[j]){
r=r-2;
}
}
}  
}
printf("%d",r);
return 0;
}