/* package whatever; // don't place package name! */
import java.util.*;
import java.lang.*;
import java.io.*;
/* Name of the class has to be "Main" only if the class is public. */
class Main
{
public static boolean isPossible(int[] t, int[] d, int w, int m){
int n = t.length;
long lit = 0;
for(int i=0;i<n;i++){
if(m == 9){
System.
out.
println((m
/d
[i
])*t
[i
]); }
lit += (((m-1)/d[i])+1)*t[i];
}
// System.out.println(m + " " + lit);
return lit>=w;
}
{
// your code goes here
Scanner sc
= new Scanner
(System.
in); int w = sc.nextInt();
int n = sc.nextInt();
int[] t = new int[n];
int[] d = new int[n];
for(int i=0;i<n;i++){
t[i] = sc.nextInt();
}
for(int i=0;i<n;i++){
d[i] = sc.nextInt();
}
int l = 0;
int r = (int)1e9;
int ans = 0;
while(l <= r){
int m = l+ (r-l)/2;
if(isPossible(t,d,w,m)){
ans = m;
r = m - 1;
}
else{
l = m + 1;
}
}
}
}
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