#include <bits/stdc++.h>
using namespace std;
const int maxn = 5e5 + 100;
int n;
long double X[maxn], V[maxn];
inline int read() {
  int first = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    first = first * 10 + ch - '0';
    ch = getchar();
  }
  return first * f;
}
bool chk(long double t) {
  int cur[n + 1];
  for (int i = 1; i <= n; i++) cur[i] = i;
  for (int i = 1; i <= n - 1; i++)
    if (X[cur[i]] + V[cur[i]] * t >= X[cur[i + 1]] + V[cur[i + 1]] * t) {
      if (V[cur[i]] > 0 && V[cur[i + 1]] < 0) return true;
      swap(cur[i], cur[i + 1]);
    }
  for (int i = n; i >= 2; i--)
    if (X[cur[i]] + V[cur[i]] * t <= X[cur[i - 1]] + V[cur[i - 1]] * t) {
      if (V[cur[i]] < 0 && V[cur[i - 1]] > 0) return true;
      swap(cur[i], cur[i - 1]);
    }
  return false;
}
int main() {
  n = read();
  for (int i = 1; i <= n; i++) {
    X[i] = read();
    V[i] = read();
  }
  long double l = 0, r = 1e9;
  bool check = false;
  for (int i = 0; i < 100; i++) {
    long double mid = (l + r) / 2;
    if (chk(mid)) {
      check = true;
      r = mid;
    } else
      l = mid;
  }
  if (!check)
    cout << "-1\n";
  else
    cout << setprecision(15) << fixed << l << "\n";
  return 0;
}
