#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>

using namespace std;

int main(){
    int n,S[1000],a,b,maxS=0,ans=0;
    
    scanf("%d",&n);
    
    for(int i=0;i<n;i++){
        scanf("%d%d",&a,&b);
        
        S[i]=a+4*b;
        maxS=max(maxS,S[i]);
    }
    
    for(int i=0;i<n;i++){
        if((maxS-S[i])%2){
            puts("-1");
            return 0;
        }
        
        ans+=(maxS-S[i])/2;
    }
    
    
    printf("%d\n",ans);
    
    
    return 0;
}