public static int evalDigit() throws InvalidExpression{


            if (e[i]>='0' && e[i]<='9'){
              int result = e[i]-(int)'0';
              i++;
              return result;
            } else {
              throw new InvalidExpression("index "+i);
            }
          }


public static int evalTerm() throws InvalidExpression{

                        return evalDigit();
          }

public static int evalExpr() throws InvalidExpression {

            int result=evalTerm();
            while (e[i]=='+' || e[i]=='-'){
              switch (e[i]){
                case '+':
                  i++;
                  result += evalTerm();
                  break;
                case '-':
                  i++;
                  result -= evalTerm();
                  break;
              }
            }
            return result;
          }