#include <fstream>
using namespace std;
ifstream f("raft.in");
ofstream g("raft.out");
int d,r,c,rr,i,x1,x2,y,n,x3,h[500],sh[500];
int main()
{
    f >> c >> rr >> n;
    for(i = 1; i <= n; ++i)
    {
        f >> d >> r;
        if(r > h[d])
            h[d] = r;
    }
    sh[1] = h[1];
    sh[2] = max(h[1], h[2]);
    sh[3] = max(sh[2], h[3]);
    for(i = 4; i <= c; ++i)
    {
        x1 = h[i] + sh[i - 1];
        x2 = max(h[i], h[i - 1]) + sh[i - 2];
        y = x2 - sh[i - 2];
        x3 = max(y, h[i - 2]) + sh[i - 3];
        sh[i] = min(x1, x2);
        sh[i] = min(sh[i], x3);
    }
    g << sh[c] << '\n';
    return 0;
}