eraser

#include <bits/stdc++.h>
using namespace std;
#define INF 2e18

using ll = long long;
ll pro[40];

int main() {

    ll n, k, c; scanf("%lld%lld%lld", &n, &k, &c);

    pro[0] = c;
    for(int i=1;i<=40; ++i) pro[i] = INF;
    int mx = 0;
    while(k--){
        int a, b; scanf("%d%d", &a,&b);
        pro[a] = min(pro[a], (ll)b);
        mx = max(mx, a);
    }

    for(int i=1; i<=mx; ++i) {
        ll x = 2;
        for(int j=i-1; j>=0; --j, x *= 2) {
            pro[i] = min(pro[i], pro[j]*x);
        }
    }
    ll res = 0;
    for(int i=mx; i>=0 && n; --i) {
        ll k = (1 << i);
        res += pro[i]*(n/k);
        // printf("%d\n", n/k);
        n -= k*(n/k);
    }

    printf("%lld", res);
    return 0;
}