#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int beaninbag,bag,numoffam;
    int age[120]={0};
    scanf("%d %d %d",&beaninbag,&bag,&numoffam);
    int totalbean=beaninbag*bag;
    for(int i=0;i<numoffam;i++)
    {
        scanf("%d",&age[i]);
    }
    for(int i=0;i<numoffam;i++)
    {
        totalbean-=age[i];
        if(totalbean<0)
        {
            break;
        }
    }
    if(totalbean<0)
    {
        printf("-1");
    }
    else
    {
        printf("%d",totalbean);
    }
    
}