class Solution {
public:
    int getNumber(string& expression, int& pos){
        int result = 0;
        while((isdigit(expression[pos]) || expression[pos] == ' ') && pos < expression.size()){
            if(isdigit(expression[pos])){
                result = result * 10 + (expression[pos] - '0');
            }
            pos++;
        }
        return result;
    }

    int parseTerm(string& expression, int& pos){
        int result = getNumber(expression, pos);
        
        while(pos < expression.size() && (expression[pos] == '*' || expression[pos] == '/')){
            char op = expression[pos++];
            int nextNumber = getNumber(expression, pos);
            if(op == '*'){
                result *= nextNumber;
            } else if(op == '/'){
                result /= nextNumber;
            }
        }
        return result;
    }

    int parseExpression(string& expression, int& pos){
        int result = parseTerm(expression, pos);

        while(pos < expression.size() && (expression[pos] == '+' || expression[pos] == '-')){
            char op = expression[pos++];
            int nextTerm = parseTerm(expression, pos);
            if(op == '+'){
                result += nextTerm;
            } else if(op == '-'){
                result -= nextTerm;
            }
        }
        return result;
    }

    int calculate(string s) {
        int pos = 0;
        return parseExpression(s, pos);
    }
};