#include <bits/stdc++.h> using namespace std; int w[10001], h[10001], sum[10001], flag[10001]; int main(){ int n, m, num = 0; cin >> n; int cnt = 0; for(int i = 1; i <= n; i++){ cin >> w[i] >> h[i]; } cin >> m; for(int i = 1; i <= n; i++){ sum[i] = w[i] * h[i]; sum[i] /= m; if(sum[i] <= 0){ flag[i] = 0; cnt++; }else{ flag[i] = 1; } } for(int i = 1; i <= n; i++){ num = min(sum[i], sum[i - 1]); } if(cnt == 0){ cout << num; }else{ cout << "-1"; } return 0; }