#include <cstdio>

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

  int flag = -1;
  long int max = 0;
  long int length[1000];
  for (int i = 0; i < n; i++) {
    long int a, b;
    scanf("%ld %ld", &a, &b);

    if (flag == -1) {
      flag = a % 2;
    } else {
      if (flag != a % 2) {
        printf("-1\n");
        return 0;
      }
    }

    length[i] = a + 4 * b;
    if (max < length[i]) max = length[i];
  }

  long int answer = 0;
  for (int i = 0; i < n; i++) {
    answer += (max - length[i]) / 2;
  }
  printf("%ld\n", answer);

  return 0;
}