string change(string a, int m, int n) {
int remainder = 0;
int len = a.size();
string b;
for (int i = 0; i < len;) {
remainder = 0;
for (int j = i; j < len; ++j) {
int current = (remainder * m + a[j] - '0') % n;
a[j] = (remainder * m + a[j] - '0') / n + '0';
remainder = current;
}
b += (remainder + '0');
while (a[i] == '0') i++;
}
reverse(b.begin(), b.end());
return b;
}