杭电acm2111

#include <stdio.h>
#include <algorithm>

using namespace std;

struct jewelry {
int price;
int volume;

bool operator < (const jewelry &A) const {
return price > A.price;
}

}buf[101];

int main() {

int v, n, i, ans, ids;
while(~scanf("%d", &v) && v) {
scanf("%d", &n);
for(i=0; i<n; i++)
scanf("%d%d", &buf[i].price, &buf[i].volume);
sort(buf, buf+n);

ids = 0;
ans = 0;
while(v>0 && ids<n) {

if(v>=buf[ids].volume) {
v -= buf[ids].volume;
ans += buf[ids].price * buf[ids].volume;
} else {
ans += v * buf[ids].price;
v = 0;
}
ids ++;
}

printf("%d\n", ans);
}

return 0;
}