#include <bits/stdc++.h>

using namespace std;

typedef vector<int> vi;
typedef vector<vi> vii;
typedef vector<string> vs;
typedef vector<bool> vb;
typedef vector<vb> vbb;
typedef pair<int, int> pii;
typedef long long ll;
typedef unsigned long long ull;

#define all(a)  (a).begin(),(a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair

#define loop(i,a,b) for(int i=(a);i<ull(b);++i)
#define rep(i,n) loop(i,0,n)

const double eps = 1e-10;
const double pi  = acos(-1.0);
const double inf = (int)1e8;

/*素数判定*/
const int MAX = 1000100;
bool prime[MAX];

void sieve(){
    fill(prime, prime+MAX, true);
    prime[0] = prime[1] = false;

    for(int i=0; i < MAX; i++){
            if(!prime[i]) continue;
            for(int j=i+i; j < MAX; j += i){
                        prime[j] = false;
                    }
        }
}

int main(){
    sieve();
    int n;
    while(cin >> n, n){
        int ans = 0;
        for(int i=0; i< n; i++){
            int a, b;
            cin >> a >> b;
            for(int j = a-b; j <= min(a+b, 999983); j++){
                if(prime[j]) ans++;
            }
            ans--;
        }
        cout << ans << endl;
    }
}