#include <iostream>
#include <vector>
#include <set>
using namespace std;
int main() {
int a, b, how_many, i,j;
bool divided;
cin >> a >> b;
if(a>b){
i=a; a=b; b=i;
}
how_many = 0;
for (int i = a+1; i < b; i++) {
divided = false;
for (j=a; j<=(i/2);j++) {
if (i % j == 0) {
divided = true;
break;
}
}
if(!divided) how_many++;
}
cout << how_many << '\n';
return 0;
}
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