#include <bits/stdc++.h>
using namespace std;
#define mp make_pair
#define pb push_back
typedef long long int LLI;
typedef vector<int> vi;
typedef pair<int,int> pii;

int poss[200001];
int dp[200001][1000];
int main() {
    int i;
    int n,a,b;
    scanf("%d",&n);
    fill(poss,poss+2*n+1,-1);
    for (i = 0; i < n; i++) scanf("%d %d",&a,&b),poss[b] = a;

    int j,B = 1000;
    for (i = 0; i < B; i++) dp[0][i] = (i == 0) ? 0:1e9;
    for (i = 1; i <= 2*n; i++) {
        for (j = 0; j < B; j++) dp[i][j] = (j == 0) ? i:dp[i-1][j];
        if (poss[i] != -1) {
            for (j = 0; j < B; j++) {
                if (i-1-dp[i-1][j] <= poss[i]) break;
            }
            int c = j;
            for (j = 0; j+c+1 < B; j++) {
                dp[i][j+c+1] = min(dp[i][j+c+1],dp[poss[i]][j]);
                if (dp[poss[i]][j] == 1e9) break;
            }
        }
    }
    for (i = 0; i < B; i++) {
        if (dp[2*n][i] == 1e9) break;
    }
    printf("%d\n",i-1);

    return 0;
}