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

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

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

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

    return 0;
}