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Cod sursa(job #415879)
#include<cstdio>
#include<vector>
#include<algorithm>
#define pb push_back
#define mp make_pair
#define f first
#define s second
const int mod = 1000013;
const int maxn = 1050;
using namespace std;
vector <pair<pair<int,int>,int> > hash[mod];
int i , j , n , l , v[maxn] , sol , cnt , k , sum , ind;
int main()
{
freopen("oite.in","r",stdin);
freopen("oite.out","w",stdout);
scanf("%d %d",&n,&l);
for( i = 1 ; i <= n ; ++i )
scanf("%d",&v[i]);
for ( i = 1 ; i <= n ; ++i )
for( j = i + 1 ; j <= n ; ++j )
hash[(v[i] + v[j]) - (( v[i] + v[j] ) / mod )* mod].pb(mp(mp(i,j),v[i] + v[j]));
for ( i = 1 ; i <= n ; ++i )
for( j = i + 1 ; j <= n ; ++j ) {
sum = (l - (v[i] + v[j])) ;
if ( sum < 0 ) continue;
ind = sum - (sum / mod * mod);
cnt = 0;
for( k = 0 ; k < hash[ind].size() ; ++k )
if( hash[ind][k].s == sum && j < hash[ind][k].f.f)
cnt++ ;
sol += cnt;
}
//printf("%d\n",sol);
printf("%d\n",sol );
return 0;
}