Rust got NZEC in ANARC05H

hi all.

i solved ANARC05H with c++. but i got nzec when implemented it with rust.

this is my rust code. can anyone tell me what is wrong with this code? i'm new in rust.

use std::io;

fn solve(position: usize, previous_accumulation: i32, inp_length: usize, inp: &String) -> i32 {
	if position >= inp_length { return 1; }

	let mut result: i32 = 0;
	let mut accumulation: i32 = 0;
	let mut index: usize = position;

	while index < inp_length {
		accumulation += inp.chars().nth(index).unwrap().to_digit(10).unwrap() as i32;
		index += 1;

		if accumulation >= previous_accumulation {
			result += solve(index, accumulation, inp_length, inp);
		}
	}

	return result;
}

fn main() {
	let mut inp: String = String::new();
	let mut num: i32 = 0;

	loop {
		inp.clear();
		io::stdin().read_line(&mut inp);

		let first_char = inp.chars().nth(0).unwrap();
		if !first_char.is_digit(10) { break; }

		let inp_length = inp.chars().count() - 1;

		num += 1;
		println!("{}. {}", num, solve(0, 0, inp_length, &inp));
	}
}

thanks a lot.