#include <cstdio>
#include <algorithm>

int main(){
    int N;
    scanf("%d", &N);

    int L[1000], mx = 0;
    for(int i=0;i<N;i++){
        int a, b;
        scanf("%d %d", &a, &b);

        L[i] = a + 4 * b;
        mx = std::max(mx, L[i]);
    }

    int x = L[0] & 1;
    long long sum = 0;
    for(int i=0;i<N;i++){
        if((L[i] & 1) != x){puts("-1"); return 0;}
        sum += (mx - L[i]) / 2;
    }

    printf("%lld\n", sum);
}