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