为什么WA+TLE了？

#include <bits/stdc++.h>
#include <iostream>
#include <stdio.h>

using namespace std;

int a [ 20000 + 100 ], b [ 20000 + 100 ];
int sum = 0;

int isprime ( int s )
{
	if ( s == 0 || s == 1 )
	{
		return 0;
	}
//	int m;
//	m = sqrt ( s );
	for ( int i = 2; i <= s; i ++ )
	{
		if ( s % i == 0 )
		{
			return 0;
		}
	}
	return 1;
}

int main ()
{
	ios::sync_with_stdio ( 0 );
	cin.tie ( 0 ), cout.tie ( 0 );
	int n, m;
	cin >> n >> m;
	for ( int i = 0; i < n; i ++ )
	{
		cin >> a [ i ];
	}
	for ( int j = 0; j < m; j ++ )
	{
		cin >> b [ j ];
	}
	for ( int i = 0; i < n; i ++ )
	{
		for ( int j = 0; j < m; j ++ )
		{
			int s = a [ i ] * b [ j ];
			if ( s <= n + m && isprime ( s ) == 1 )
			{
				sum ++;
			}
		}
	}
	cout << sum << endl;
	return 0;
}