string subtract(string a, string b) {
    string ans="";
    reverse(a.begin(), a.end());
    reverse(b.begin(), b.end());
    int carry=0;
    for (long long i=0; i<a.length(); i++) {
        int sub;
        if (i>=b.length()) sub=a[i]-'0'-carry;
        else sub=a[i]-b[i]-carry;
        if (sub<0) {
            sub+=10;
            carry=1;
        }
        else carry=0;
        ans.push_back(sub+'0');
    }
    while (ans.back()=='0') ans.pop_back();
    reverse(ans.begin(), ans.end());
    return ans;
}