[ l , r ] will be the range from total [ c , d ] x + y belongs For any r such that r = x + y We can choose z , in the range [ c , d ] only less than r , Thus total permutation be
int x1 = ( r * ( r + 1 ) ) / 2 ; int x2 = ( l * ( l + 1 ) ) / 2 ; ans += x1 - x2 ; } return ans ; }