To solve this problem, we need to keep track of two things while we traverse the lists from 1 to n:

- total : Total of A[i] - B[i] for all values of i from 1 to n
- start: the last position which will total up to a positive value till the end, if counting is started from that position.

If total is negative,

return -1;

else,

return start;

Code: