#include <iostream>
int main(void){
    int a,b,c;
    std::cin>>a>>a>>b;
    c=b-a;
    for(;std::cin>>a>>b;){
        if(b-a-c){c=0;break;}
    }
    std::cout<<(c>0?c:-1)<<"\n";
    return 0;
}