#include<iostream>
#include<cstring>
#include<cstdio>
using namespace std;
int main(){
    int n,maxn;
    int N[105],sum1,sum2,sum3=0,NUM1[20000],NUM2[20000];
    memset(NUM2,0,sizeof(NUM2));
    cin>>n;
    for(sum1=0;sum1<n;sum1++){
        cin>>N[sum1];
        NUM2[N[sum1]]=1;
    }


    for(sum1=0;sum1<n;sum1++){
        for(sum2=sum1+1;sum2<n;sum2++){
            NUM1[N[sum1]+N[sum2]]=1;
            maxn=max(maxn,N[sum1]+N[sum2]);
            }
        }
    for(sum1=0;sum1<maxn;sum1++){
        if(NUM1[sum1]+NUM2[sum1]==2){
            sum3++;
        }
    }
    cout<<sum3;
    return 0;
    
}