#include <bits/stdc++.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>
 
using namespace std;
 
///////////////////////////////////////////////////////////////////////////////
 
#define DEBUG 0
 
#define pb push_back
#define V vector
#define M unordered_map
#define S static
#define PI  (3.1415926535)
 
#define rep(i,n) for(ll i=0LL; i<n; ++i)
#define srep(i,s,n) for(ll i=s; i<n; ++i)
#define rrep(i,n) for(ll i=n-1LL; i>=0LL; --i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define CIN(x) do { \
        assert(!cin.eof()); \
        cin >> x; \
        assert(!cin.fail()); \
} while(0);
#if DEBUG
#define debug_print(...) _debug_print(__VA_ARGS__)
#define debug_printf(...) printf(__VA_ARGS__)
#define debug_print_time _debug_print_time
#else // DEBUG
#define debug_print(...)
#define debug_printf(...)
#define debug_print_time
#endif // DEBUG
 
typedef long long ll;
typedef unsigned long long ull;
typedef tuple<ll, ll> t2;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;
typedef tuple<ll, ll, ll, ll, ll> t5;
template <typename T>
using priority_queue_incr = priority_queue<T, V<T>, greater<T> >;
 
///////////////////////////////////////////////////////////////////////////////
 
template <typename TT, typename T>
T get_m(M<TT, T> &m, TT k, T default_value)
{
    if (m.find(k) == m.end()) return m[k] = default_value;
    return m[k];
}
 
template <typename TT, typename T>
void incr_m(M<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}
 
void llin(ll &a)
{
    CIN(a);
}
 
void llinl1(V<ll> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a;
        CIN(a);
        v.push_back(a);
    }
}
 
void llinl2(V<t2> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b;
        CIN(a >> b);
        v.push_back(t2(a, b));
    }
}
 
void llinl3(V<t3> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c;
        CIN(a >> b >> c);
        v.push_back(t3(a, b, c));
    }
}
 
void llinl4(V<t4> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c, d;
        CIN(a >> b >> c >> d);
        v.push_back(t4(a, b, c, d));
    }
}
 
void llina(V<ll> &v, ll count)
{
    llinl1(v, count);
}
 
template <typename T>
void sort(V<T> &v)
{
    sort(v.begin(), v.end());
}
 
template <typename T>
void sort_reverse(V<T> &v)
{
    sort(v.begin(), v.end(), greater<T>());
}
 
template <typename T>
void _debug_print(T x)
{
    cout << x << endl;
}
 
void _debug_print(const t2 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    cout << "-- " << x1 << " -- " << x2 << endl;
}
 
void _debug_print(const t3 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    cout << "-- " << x1 << " -- " << x2 << " -- " << x3 << endl;
}
 
void _debug_print(const t4 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    ll x4 = get<3>(x);
    cout << "-- " << x1 << " -- " << x2
         << " -- " << x3 << " -- " << x4 << endl;
}
 
template <typename T>
void _debug_print(T xarray[], ll n)
{
    rep (i, n) _debug_print(xarray[i]);
}
 
template <typename T>
void _debug_print(const V<T> &xlist)
{
    for (auto x : xlist) {
        cout << "-- ";
        _debug_print(x);
    }
}
 
template <typename T>
void _debug_print(const set<T> &xset)
{
    for (auto x : xset) {
        cout << "-- ";
        _debug_print(x);
    }
}
 
template <typename TT, typename T>
void _debug_print(const M<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}
 
void _debug_print_time(const char *prefix)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
    struct tm *tm = localtime(&tv.tv_sec);
    printf("-- %s %02d:%02d:%02d.%06ld\n",
           prefix, tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
}
 
///////////////////////////////////////////////////////////////////////////////
 
void _main();
int main()
{
    cout << setprecision(12);
#if !DEBUG
    ios::sync_with_stdio(false);
    cin.tie(0);
#endif
 
    _main();
    return 0;
}
 
void solve(ll &cnt, const V<ll> &alist, ll n, ll border)
{
    debug_printf("------------------ solve border=%lld\n", border);
 
    for (auto a1 : alist) {
 
        ll left = 0;
        ll right = n-1;
        ll now = (left+right)/2LL;
        ll lastidx = -1;
 
        for ( ; left <= right; now = (left+right)/2LL) {
            ll sum = a1 + alist[now];
            if (sum > border) {
                lastidx = max(lastidx, now);
                left = now + 1LL;
            }
            else {
                right = now - 1LL;
            }
        }
 
        debug_printf("a1=%lld lastidx=%lld\n", a1, lastidx);
 
        cnt += (lastidx+1);
    }
}
 
void _main()
{
    ll n; llin(n);
    ll m; llin(m);
    V<ll> alist; llina(alist, n);
 
    sort_reverse(alist);
 
    ll border = alist[0] * 2LL;
    ll left = 0;
    ll right = alist[0] * 2LL;
    ll now = (left + right) / 2LL;
 
    for ( ; left <= right; now = (left + right) / 2LL) {
        debug_printf("left=%lld right=%lld\n", left, right);
        ll cnt = 0;
        solve(cnt, alist, n, now);
        debug_printf("cnt=%lld m=%lld\n", cnt, m);
        if (cnt <= m) {
            border = min(border, now);
            right = now - 1LL;
        }
        else {
            left = now + 1LL;
        }
    }
 
    debug_printf("---- border = %lld\n", border);
 
    V<ll> sumlist;
    ll sum = 0;
    sumlist.pb(sum);
    for (auto a : alist) {
        sum += a;
        sumlist.pb(sum);
    }
 
    auto end = alist.rend();
    ll cnt = 0;
    ll ans = 0;
 
    for (auto a1 : alist) {
        auto iter = upper_bound(RALL(alist), border - a1);
        ll lcnt = end - iter;
        cnt += lcnt;
        ans += sumlist[lcnt] + a1 * lcnt;
    }
 
    assert(cnt <= m);
    ans += (m - cnt) * border;
 
    debug_printf("---- ans\n");
    cout << ans << endl;
}

Measured memory_bytes: 5604