#include #define rep(i,a,b) for(int i=a;i<(b);++i) #define erep(i,a,b) for(int i=a;i<=(int)(b);++i) #define per(i,a,b) for(int i=(a);i>(b);--i) #define eper(i,a,b) for(int i=(a);i>=b;--i) #define pb push_back #define mp make_pair #define INF (1<<30)-1 #define MOD 1000000007 #define all(x) (x).begin(),(x).end() #define vii vector #define vll vector using namespace std; typedef long long ll; typedef pair Pii; templatebool chmax(T &a, const T &b) { if (abool chmin(T &a, const T &b) { if (b> n >> x; vll b(n); map a; rep(i, 0, n) { cin >> b[i]; a[b[i]]++; } rep(i, 0, n) { ans += a[x-b[i]]; } cout << ans << endl; return 0; }