#include <stdio.h>
#include <stdlib.h>

int func(const void *a, const void *b){
    return *(int *)a - *(int *)b;
}

int main(){
    long long int N, i;
    long long int a[1000], b[1000], width[1000];
    long long int cnt = 0, sub, divi;

    scanf("%lld\n", &N);
    for(i = 0; i < N; i++){
        scanf("%lld %lld\n", &a[i], &b[i]);
        width[i] = a[i] + 4*b[i];
    }
    qsort(width, N, sizeof(long long int), func);
    /*
    for(i = 0; i < N; i++){
        printf("%lld\n", width[i]);
    }
    */
    for(i = 0; i < N; i++){
        sub =  width[N - 1] - width[i];
        //printf("max %lld\n", width[N - 1]);
        if(sub > 0){
            divi = sub/2;
            if(sub%2 == 0){
                cnt += divi;
            }else{
                printf("-1\n");
                return 0;
            }
        }
    }
    printf("%lld\n", cnt);

    return 0;
}