Spoj candies and milestones

//http://www.spoj.com/problems/CANDYSTN/

#include<stdio.h>

//fast input
inline int fastRead_int() {
    register int c = getchar_unlocked();
    int x = 0;
    int neg = 0;
    for(; ((c<48 || c>57) && c != '-'); c = getchar_unlocked());
    if(c=='-') {
    neg = 1;
    c = getchar_unlocked();
    }
    for(; c>47 && c<58 ; c = getchar_unlocked()) {
    x = (x<<1) + (x<<3) + c - 48;
    }
    if(neg)
    x = -x;
   
    return x;
}


int main(){
long long int t,n,m,ar[100000],i,j,p,q;
t=fastRead_int();
while(t--){
p=0;q=0;
long long min=0,minb=0;
n=fastRead_int();
m=fastRead_int();
ar[0]=fastRead_int();
for(i=1;i<m;i++){
ar[i]=fastRead_int();
if(ar[i]>ar[i-1]){
p-=(ar[i]-ar[i-1]);
q+=(ar[i]-ar[i-1]);
if(p<minb){
minb=p;
}
}else if(ar[i]<ar[i-1]){
p+=(ar[i-1]-ar[i]);
q-=(ar[i-1]-ar[i]);
if(q<min){
min=q;
}
}else{
q--;
if(q<min){
min=q;
}
}
}
j=(-1)*minb+(-1)*min+1;
if(j>n){
printf("-1\n");
}else{
printf("%lld\n",(-1)*min+1);
}
}
return 0;
}