how many integers [A-B] digit sum equal to x (digit dp)

#include <bits/stdc++.h>

using namespace std;
#define ll long long int
int dp[12][2][95];
string s;
int x;

int solve(int pos, int isSmall, int digitSum) {
    if (pos == s.size()) {
        return (digitSum == x);
    }
    if (dp[pos][isSmall][digitSum] != -1) {
        return dp[pos][isSmall][digitSum];
    }
    int low = 0, high = s[pos] - '0';
    if (isSmall) {
        high = 9;
    }
    int res = 0;
    for (int i = low; i <= high; i++) {
        int new_sm = isSmall | (i < high);
        int val = solve(pos + 1, new_sm, digitSum + i);
        res += val;
    }
    return dp[pos][isSmall][digitSum] = res;
}

int main() {
    memset(dp, -1, sizeof(dp));
    cin >> s >> x;
    cout << solve(0, 0, 0);
    return 0;
}