#include <bits/stdc++.h>
using namespace std;
#define int long long
#define arr array
#define vec vector
const int N = 2e5 + 5, LG = 22, INF = 1e12;
int n, m;
arr<int, N> l, r;
arr<arr<vec<int>, LG>, N> jmp;
void jmp_cmp() {
jmp[0][0] = {0, 0};
for (int i = 1; i <= n; i++) {
jmp[i][0] = {0, 0};
for (int j = 1; j <= n; j++) {
if (i == j) continue;
if (l[i] <= r[i] && l[j] <= r[j]) {
if (l[j] < l[i] || l[j] > r[i]) continue;
if (r[j] <= r[i]) continue;
jmp[i][0] = max(jmp[i][0], {r[j] - r[i], j});
} else if (l[i] <= r[i] && l[j] > r[j]) {
if (l[j] < l[i] || l[j] > r[i]) continue;
jmp[i][0] = max(jmp[i][0], {r[j] + m - r[i], j});
} else if (l[i] > r[i] && l[j] <= r[j]) {
if (l[j] > r[i]) continue;
if (r[j] <= r[i]) continue;
jmp[i][0] = max(jmp[i][0], {r[j] - r[i], j});
} else {
if (l[j] < l[i]) continue;
if (r[j] <= r[i]) continue;
jmp[i][0] = max(jmp[i][0], {r[j] - r[i], j});
}
}
}
// for (int i = 1; i <= n; i++) {
// cout << i << ": " << jmp[i][0][0] << " " << jmp[i][0][1] << '\n';
// }
for (int i = 1; i <= 20; i++) {
for (int j = 0; j <= n; j++) {
vec<int> x = jmp[j][i - 1];
vec<int> y = jmp[x[1]][i - 1];
jmp[j][i] = {x[0] + y[0], y[1]};
}
}
}
// i changes
int lft(int i) {
int rq = m - (r[i] - l[i]);
// cout << i << ": " << rq << '\n';
int cr = 0, ans = 1;
for (int j = 20; j >= 0; j--) {
vec<int> x = jmp[i][j];
if (x[1] == 0) continue;
if (cr + x[0] >= rq) continue;
i = x[1], cr += x[0], ans += (1 << j);
}
vec<int> x = jmp[i][0];
if (x[1] == 0) return INF;
ans++;
return ans;
}
void ans_cmp() {
int ans = INF;
for (int i = 1; i <= n; i++)
ans = min(ans, lft(i));
cout << ((ans == INF) ? -1 : ans) << '\n';
}
signed main() {
// freopen("in", "r", stdin);
cin >> n >> m;
for (int i = 1; i <= n; i++) cin >> l[i] >> r[i];
jmp_cmp();
ans_cmp();
}
