Implement a function that returns the sum of every integer from 1 to n by recursion, with the sum for 0 defined as 0.

• Solve this with recursion, not a loop: sum_to_n(n) is n + sum_to_n(n - 1).
• The base case is sum_to_n(0), which returns 0.
• Check the sign before you recurse. A negative n never reaches the base case, so a missing guard runs forever.
• The contract covers n from 0 to 65535. At 65535 the sum is 2147450880, the last total that still fits in an int; nothing above that is defined and nothing above that is tested.
• Keep in mind that sum_to_n(65535) opens 65535 nested frames before the first one returns.
• A negative n is invalid input: return -1.

Implement a function that returns the sum of every integer from 1 to n by recursion, with the sum for 0 defined as 0.

• Solve this with recursion, not a loop: sum_to_n(n) is n + sum_to_n(n - 1).
• The base case is sum_to_n(0), which returns 0.
• Check the sign before you recurse. A negative n never reaches the base case, so a missing guard runs forever.
• The contract covers n from 0 to 65535. At 65535 the sum is 2147450880, the last total that still fits in an int; nothing above that is defined and nothing above that is tested.
• Keep in mind that sum_to_n(65535) opens 65535 nested frames before the first one returns.
• A negative n is invalid input: return -1.