import java.util.*;
class Main{
    public static void main(String[] args){
        Scanner s=new Scanner (System.in);
        int n=s.nextInt();
        
        int score=0;
        for (int i=0;i<n;i++){
            if(!s.hasNextInt()){
                System.out.println(-1);
            }
            int act=s.nextInt();
            int points=s.nextInt();
            if (act<1 || act>2 ){
                System.out.println(-1);
                return;
                
            }
            if(act==1){
                score+=points;
            }
            else{
                score-=points;
            }
            
        }
        if(score=0){
            System.out.println("Invalid input");
        }
        else{
            System.out.println(score);
        }
    }
}