#include <iostream>
#include <stdio.h>
using namespace std;

int main(int argc, char const* argv[])
{
    int n;
    cin>>n;
    int x[n],y[n],z[n];

    int counter=1;
    for (int i = 0; i < n; i++) {
        cin>>x[i]>>y[i];
        z[i] = y[i] - x[i];
    }

    for (int i = 0; i < n-1; i++) {
        for (int j = i+1; j < n; j++) {
            if(z[j]==z[i]){
                counter++;
            }
        }
    }
    if(counter==1){
        cout<<"-1"<<endl;
        return 0;
    }
    cout<<counter<<endl;
    return 0;
}