Given a list of integers from 1 to n with exactly one number missing, find and return the missing number. The parameter n represents the total count of numbers in the complete sequence. Use the mathematical sum formula for optimal O(n) time complexity and O(1) space complexity: sum of 1 to n = n*(n+1)/2.
Examples:
Input: nums = [1, 2, 4, 5], n = 5
Output: 3
Explanation: The sequence should be 1 to 5, but 3 is missing. Using formula: expected sum = 5*6/2 = 15, actual sum = 12, missing = 3
Input: nums = [2, 3, 4, 5, 6, 7, 8, 9, 10], n = 10
Output: 1
Explanation: The sequence should be 1 to 10, but 1 is missing. Expected sum = 10*11/2 = 55, actual sum = 54, missing = 1
Input: nums = [1, 2, 3, 4, 5, 6, 7, 8, 9], n = 10
Output: 10
Explanation: The last number in the sequence is missing
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