long long n,m;a[1010],s;main(p){for(gets(&p);~scanf("%d%d",++s+a,&p);m=a[s]>m?a[s]:m)a[s]+=p*4;for(;--s&&~n%2;n+=m-a[s]);s=!printf("%lld",n&1?-1:n/2);}