#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <cstring>

typedef long long ll;
using namespace std;

#define mod 1000000007
#define INF 1000000000 //(int)1e9
#define LLINF 2000000000000000000 //(ll)2e18
#define PI  3.1415926536

#define SIZE 1000


int main(){
    int n;
    int x,y,ans;
    
    scanf("%d",&n);
    
    scanf("%d%d",&x,&y);
    
    ans = y-x;
    
    if(ans<1){
        puts("-1");
        return 0;
    }
    
    for(int i=1;i<n;i++){
        scanf("%d%d",&x,&y);
        if(ans != y-x){
            puts("-1");
            return 0;
        }
    }
    
    printf("%d\n",ans);
    
    return 0;
}