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Cod sursa(job #227394)
program pascal;
var f,g:text;
v:array[1..800] of integer;
n,i,j,m,t,r:integer; nr:longint;
procedure quicksort(s,d:integer);
var a,b,aux,ia:integer;
begin
a:=s; b:=d;
repeat
while v[a]<v[b] do b:=b-1;
aux:=v[a];
v[a]:=v[b];
v[b]:=aux;
ia:=1; a:=a+1;
if a<b then
begin
while v[a]<v[b] do a:=a+1;
if a<>b then
begin
aux:=v[a];
v[a]:=v[b];
v[b]:=aux;
ia:=0;
b:=b-1;
end;
end;
until a>=b;
if s<a-ia then quicksort(s,a-ia);
if a-ia+1<d then quicksort(a-ia+1,d);
end;
function caut(x,y:integer):integer;
begin
m:=(x+y) div 2;
caut:=0;
while (x<=y) and (caut=0) do
if ((v[m]<=t) and (v[m+1]>t)) or ((v[m]<=t) and (m=n)) then caut:=m
else
if (t>=v[m]) and (v[m+1]<=t) then caut:=caut(m+1,y)
else caut:=caut(x,m-1);
end;
begin
assign(f,'nrtri.in'); reset(f);
assign(g,'nrtri.out'); rewrite(g);
readln(f,n);
for i:=1 to n do read(f,v[i]);
quicksort(1,n);
nr:=0;
for i:=1 to n-1 do
for j:=i+1 to n-1 do
begin
t:=v[j]+v[i];
r:=caut(1,n); {cate numere is mai mici sau egale ca v[i]+v[j]}
if r>=j+1 then nr:=nr+r-(j+1)+1;
end;
write(g,nr);
close(f);
close(g);
end.