#include <bits/stdc++.h>
using namespace std;
using int64 = long long;
using i128 = __int128_t;
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
int64 A, B;
int n;
if(!(cin >> A >> B >> n)) return 0;
vector<pair<int64,int64>> intervals(n);
for(int i=0;i<n;i++){
int64 l,r; cin >> l >> r;
intervals[i] = {l,r};
}
i128 L128 = (i128)A * (i128)B;
// If L128 == 0 (shouldn't happen with A,B>=1) handle, but assume A,B>=1.
// If any interval length >= L -> all residues appear.
for(auto &it : intervals){
i128 len = (i128)it.second - (i128)it.first + 1;
if(L128 <= len){
// print L128
// cast to int64 if possible, otherwise print via i128 printing
// but constraints should keep L128 <= 1e18 normally
// we print as decimal:
i128 val = L128;
// print i128
string s;
if(val == 0) s = "0";
else {
bool neg = false;
if(val < 0){ neg = true; val = -val; }
while(val > 0){
int digit = (int)(val % 10);
s.push_back('0' + digit);
val /= 10;
}
if(neg) s.push_back('-');
reverse(s.begin(), s.end());
}
cout << s << "\n";
return 0;
}
}
// Collect residue-segments on [0, L-1]
vector<pair<int64,int64>> segs;
for(auto &it : intervals){
int64 l = it.first, r = it.second;
// compute l_mod = l % L128, r_mod = r % L128
i128 lm128 = (i128)l % L128;
i128 rm128 = (i128)r % L128;
int64 lm = (int64)lm128;
int64 rm = (int64)rm128;
if(lm <= rm){
segs.emplace_back(lm, rm);
}else{
// wraps
segs.emplace_back(lm, (int64)((i128)A*(i128)B - 1)); // L-1
segs.emplace_back(0, rm);
}
}
if(segs.empty()){
cout << 0 << "\n";
return 0;
}
// Note: L128 fits in i128. But segment endpoints are <= max(r) <= 1e18 so fit in int64.
// Sort and merge
sort(segs.begin(), segs.end());
i128 total = 0;
int64 curL = segs[0].first, curR = segs[0].second;
for(size_t i=1;i<segs.size();++i){
int64 s = segs[i].first, e = segs[i].second;
if(s <= curR + 1){
// overlap or touch
if(e > curR) curR = e;
} else {
total += (i128)curR - (i128)curL + 1;
curL = s; curR = e;
}
}
total += (i128)curR - (i128)curL + 1;
// total is <= L128 (we already ruled out any interval covering full L)
// print total as decimal
i128 val = total;
string s;
if(val == 0) s = "0";
else {
bool neg = false;
if(val < 0){ neg = true; val = -val; }
while(val > 0){
int digit = (int)(val % 10);
s.push_back('0' + digit);
val /= 10;
}
if(neg) s.push_back('-');
reverse(s.begin(), s.end());
}
cout << s << "\n";
return 0;
}
