series of natural number

Series of even natural number is given . 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ..... 10^9 you have to find a missing number if sum of first N number is given for example : N=5, Sum=10 you have to find missing term in first 5 number according to above series sum of first 5 number is 15, and if we subtract 5 from it then sum becomes 10 hence the missing number is 5 . ### Input: - First