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| #include <bits/stdc++.h>
using namespace std;
const int N = 100001;
int n, s, t, res;
vector<pair<int,int>> v;
#define MIN_INF -2e9
int main() {
scanf("%d%d%d", &s, &t, &n);
int r = MIN_INF, l = MIN_INF;
for(int i = 0; i < n; ++i) {
int a, b;
scanf("%d%d", &a, &b);
v.push_back(make_pair(a, b));
}
sort(v.begin(), v.end());
bool success = false;
for(int i = 0; i < n; ++i) {
int j = i, tmp_r = MIN_INF;
while(j < n && v[j].first <= s) {
tmp_r = max(tmp_r, v[j].second);
j++;
}
if(tmp_r < s) {
res = -1;
break;
}
res++;
if(tmp_r >= t) {
success = true;
break;
}
s = tmp_r;
i = j - 1;
}
if(!success) printf("-1");
else printf("%d", res);
return 0;
}
|