#include <bits/stdc++.h>
using namespace std;
const int MAXN = 5 * 1e5 + 10;
int n, x[MAXN], v[MAXN];
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d%d", &x[i], &v[i]);
  bool flg = 0;
  for (int i = 1; i <= n; i++)
    if (v[i] > 0) {
      for (int j = i + 1; j <= n; j++)
        if (v[j] < 0) {
          flg = 1;
          break;
        }
      break;
    }
  if (!flg) return puts("-1"), 0;
  double l = 0, r = 1e9;
  while (fabs(r - l) > 1e-10) {
    double mid = (l + r) / 2;
    double ll = 0, rr = -1e30;
    bool flg = 0;
    for (int i = 1; i <= n; i++) {
      if (v[i] > 0)
        rr = max(rr, x[i] + v[i] * mid);
      else {
        ll = x[i] + v[i] * mid;
        if (rr > ll) {
          r = mid;
          flg = 1;
          break;
        }
      }
    }
    if (!flg) l = mid;
  }
  printf("%.20lf", r);
  return 0;
}
