#include <stdio.h>

int main(void){
    int i, N, flg0 = 0, flg1 = 0;
    long long int a, b, maxlen, space = 0;
    scanf("%d", &N);
    long long int len[N];
    scanf("%lld %lld", &a, &b);
    len[0] = a + 4*b;
    maxlen = len[0];
    if(len[0]%2){
            flg0++;
        }else{
            flg1++;
        }
    for(i=1;i<N;i++){
        scanf("%d %d", &a, &b);
        len[i] = a + 4*b;
        if(len[i]%2){
            flg0++;
        }else{
            flg1++;
        }
        if(flg0>0 && flg1>0){
            printf("-1\n");
            return 0;
        }
        if(len[i]>maxlen){
            maxlen = len[i];
        }
    }

    for(i=0;i<N;i++){
        space += (maxlen - len[i]) / 2;
    }
    printf("%d\n", space);
    return 0;
}