One of the ways by which you can solve the problem is by using summation formula.the sum of 20 elements can be calculated by using a for loop from n+10  to n+110 by using n*(n+1)/2 and save it as total_sum.

You can then traverse the array again from start to end and update the value of sum as sum += a[i] and then subtract this value form total_sum to get the missing integer values.