#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <math.h>
int main(void)
{
    long long int gyou,space,tab,bigspace=0,max=0;
    long long int haba[2000]={0};
    bool alreadyprint=false;
    scanf("%lld",&gyou);
    for(int i=0;i<gyou;i++)
    {
        scanf("%lld %lld",&space,&tab);
        haba[i]=space+4*tab;
        if(haba[i]>max)
        {
            max=haba[i];
        }
    }
    for(int i=0;i<gyou;i++)
    {
        if(haba[i]%2==0&&i!=gyou-1)
        {
            if(haba[i+1]%2==1)
            {
                printf("-1");
                alreadyprint=true;
                break;
            }
        }
        else if(i!=gyou-1)
        {
            if(haba[i+1]%2==0)
            {
                printf("-1");
                alreadyprint=true;
                break;                
            }
        }
        if(max!=haba[i])
        {
            bigspace+=(max-haba[i])/2;
        }
    }
    if(alreadyprint==false)
    {
        printf("%lld",bigspace);
    }
}