import java.util.Arrays;
public class Hexadecimal {
private static final String HEXES = "0123456789abcdef";
public static int toDecimal(String input){
boolean isHex = input.matches("^[0-9a-f]+");
if (!isHex) return 0;
return Arrays.stream(input.split(""))
.map(HEXES::indexOf)
.reduce(0, (decimal, hex) -> decimal * 16 + hex);
}
}