#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// Yukicoder №178 美しいWhitespace (1) ★☆
// https://yukicoder.me/problems/no/178
// 問題文の日本語がおかしい
// ***********************
#define gc(c)	do{c = getchar_unlocked();}while(0)
#define pc(c)	putchar_unlocked(c)
	extern char getchar_unlocked(void);
	extern void putchar_unlocked(char c);
static char *GETWORD(char* str) {char c;char *cp;cp=&str[0];gc(c);while(c!=EOF){if((c==' ')||(c=='\n'))break;*cp++=c;gc(c);}*cp='\0';return &str[0];}
#define	str_len(a)	mystr_len_lim(a,9999)
static int mystr_len_lim(char *str,int lim){int i=0;do{if(str[i]=='\0')break;i++;}while(i<lim);return i;}
#define GETLINE(str)	do{char *p;fgets(str,sizeof(str),stdin);p=strchr(str,'\n');if(p)*p='\0';}while(0)
static int GETLINEINT(void) {char s[34];GETLINE(s);return atoi(s);}
static int GETWORDINT(void) {char s[34];GETWORD(s);return atoi(s);}
static long GETWORDLONG(void) {char s[34];GETWORD(s);return atol(s);}
static int max(int a,int b){if(a>b){return a;}return b;}
static int min(int a,int b){if(a<b){return a;}return b;}
#define REP(a,b) for(int a=0;a<(b);++a)
#define REP1(a,b) for(int a=1;a<=(b);++a)
void out(int n) { // 整数の表示（出力）
	int i;
	char b[30];

	if (!n) pc('0');
	else {
		if (n < 0) pc('-'), n = -n;
		i = 0; while (n) b[i++] = n % 10 + '0', n /= 10;
		while (i--) pc(b[i]);
	}
	pc('\n');
}

// *********************
// *********************
// *********************
int main( void ) {
	int	N;
	int	spc,tab,val,max;
	int	hb[1000];

	N = GETLINEINT();

	max = 0;
	REP(i,N) {
		spc = GETWORDINT();
		tab = GETWORDINT();
		val = spc + tab+ tab+ tab+ tab;
		hb[i] = val;
		if( max < val ) max = val;
	}
	spc = 0;
	REP(i,N) {
		if( max > hb[i] ) {
			val = max - hb[i];
			if( val & 1 ) { spc = -1; break; }
			spc += (val >> 1);
		}
	}
	out( spc );
}