Find the first day a shop balance goes negative

A small shop starts with some money in its account. Each day adds that day's sales and takes away its expenses, so a day's net change can be negative.

Write first_shortfall_day(start, changes) that returns the first day the balance drops below zero.

Input and output:

• start: the opening balance, an integer between 0 and 1,000,000,000.
• changes: a list of up to 100,000 daily net changes, in day order. Each is an integer between -1,000,000,000 and 1,000,000,000. It may be empty.
• Returns an integer.

Rules:

• The balance at the end of day d is start plus the first d changes.
• Days are numbered from 1, so changes[0] is day 1.
• Return the first day whose balance is below zero. A balance of exactly 0 is not below zero.
• If the balance never drops below zero, return -1.