#include<stdio.h>

int main(void){
	int n,len[1024],a,b,g,i,nlen,m=0;
	unsigned long long int r=0;
	scanf("%d",&n);
	for(i = 1;i <= n;i++){
		scanf("%d%d",&a,&b);
		nlen = a+4*b;
		len[i] = nlen;
		if(i == 1){g = nlen %2;}
		else if(nlen%2  != g){printf("-1\n");return 0;}
		if(nlen > m){m = nlen;}
	}
	for(i = 1;i <= n;i++){
		r+=(m-len[i]);
	}
	printf("%llu\n",r/2);
	return 0;
}