public class Main {
    public static void main(String[] args) {
        String s = "a3";

        char lastLetter = 0;
        boolean isDigit = false;
        int digit = 0;
        for(int i = 0; i <= s.length(); i++) {
            char c = i != s.length() ? s.charAt(i) : 0;

            if(Character.isDigit(c)) {
                isDigit = true;
                digit = digit*10 + (c-'0');
                continue;
            }
            if(isDigit) {
                for(int j = 0; j < digit-1; j++) {
                    System.out.print(lastLetter);
                }
                isDigit = false;
                digit = 0;
            }
            System.out.print(lastLetter);
            lastLetter = c;
        }
    }
}