B. Chemical table

#include <bits/stdc++.h>

using namespace std;
const int N = 4e5 + 10;
int parent[N];
set <int> group;

int Root(int u){
    if(parent[u] == u) return u;
    return parent[u] = Root(parent[u]);
}

void Merge(int u, int v){
    u = Root(u);
    v = Root(v);
    if(u == v) return;
    parent[v] = u;
}

int main(){

    int n, m, q;
    scanf("%d%d%d", &n, &m, &q);

    for(int i=1;i<=n+m;i++) parent[i] = i;

    while(q--){
        int row, col;
        scanf("%d%d", &row, &col);
        Merge(row, col + n);
    }

    for(int i=1;i<=n;i++) group.insert(Root(i));
    for(int j=1;j<=m;j++) group.insert(Root(j + n));

    printf("%d", group.size() - 1);

    return 0;
}