we will first sort the vector

sort(A.begin(),A.end());

then we will run a loop

for(int i =0;i<A.size();i++)

consider the repetition

while(i +1<A.size() and A[i]==A[i+1]

{

i++;

}

now work on real situation

if(A[i]== A.size()-1-i)

return 1;

then if we come out of loop

return -1;