1 #include<stdio.h>
2 #include<string.h>
3 #define N 50050
4 int s[N]={0},a[N],vis[N];
5 int hash(int x)
6 {
7 int y=(x%N+N)%N;
8 while(vis[y]&&s[y]!=x)
9 y=(y+1)%N;
10 return y;
11 }
12 int insert(int x)
13 {
14 int y=hash(x);
15 vis[y]=1;
16 s[y]=x;
17 }
18 int find(int x)
19 {
20 return vis[hash(x)];
21 }
22 int main()
23 {
24 int n,i,j;
25 while(~scanf("%d",&n))
26 {
27 memset(vis,0,sizeof(vis));
28 for(i=0;i<n;i++)
29 {
30 scanf("%d",&a[i]);
31 insert(a[i]);
32 }
33 int sum=0;
34 for(i=0;i<n;i++)
35 {
36 for(j=i+1;j<n;j++)
37 {
38 if(a[i]&&a[j]&&find(a[i]+a[j])==1)
39 sum++;
40 }
41 }
42 printf("%d\n",sum);
43 }
44 }