#include <bits/stdc++.h>
typedef long long i64;
using namespace std;

int main(void) {
    int n; scanf("%d", &n);
    vector<i64> v(n);
    for(int i=0; i<n; i++) {
        i64 a, b; scanf("%lld%lld", &a, &b);
        v[i] = a + 4 * b;
    }
    i64 hamark = *max_element(v.begin(), v.end());
    i64 res = 0;
    for(auto e : v) {
        i64 dif = hamark - e;
        if(dif & 1) { res = -1; break; }
        res += dif / 2;
    }
    printf("%lld\n", res);
    return 0;
}