#include <stdio.h>

int main(void){
	int i,n;
	int evenodd = 0;
	int maxSpace = 0;
	int a[1010],b[1010];
	int ans = 0;
	scanf("%d", &n);
	for(i=0;i<n;i++){
		int calc;
		scanf("%d %d", &a[i], &b[i]);
		if(a[i]%2 == 0){
			evenodd |= 1;
		}else{
			evenodd |= 2;
		}
		
		calc = a[i] + 4 * b[i];
		if(maxSpace < calc ){
			maxSpace = calc;
		}
	}
	
	if(evenodd == 3){
		ans = -1;
	}else{
		for(i=0;i<n;i++){
			int calc = a[i] + 4 * b[i];
			ans += (maxSpace - calc)/2;
		}
	}
	
	printf("%d\n", ans);
	return 0;
}