Saving HDU

#include<stdio.h>

#include<algorithm>

using namespace std;

struct bag

{

int pi;

int mi;

}a[105];

bool cmp(bag a, bag b)

{

return a.pi>b.pi;

}

int main()

{

int i, v, n, m, sum;

while(~scanf("%d", &v), v != 0)

{

scanf("%d", &n);

for(i=0; i<n; i++)

scanf("%d%d", &a[i].pi, &a[i].mi);

sum = 0;

sort(a, a+n, cmp);

for(i=0; i<n; i++)

{

if(v > a[i].mi)

{

sum = sum + a[i].pi*a[i].mi;

v = v - a[i].mi;

}

else

{

sum = sum + a[i].pi*v;

break;

}

}

printf("%d\n", sum);

}

return 0;

}