Language: C

//set many funcs template
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<stdbool.h>
#include<time.h>
#define inf 1072114514
#define llinf 4154118101919364364
#define mod 1000000007
#define pi 3.1415926535897932384

int max(int a,int b){if(a>b){return a;}return b;}
int min(int a,int b){if(a<b){return a;}return b;}
int zt(int a,int b){return max(a,b)-min(a,b);}
int round(int a,int b){if((a%b)*2 >= b){return (a/b)+1;}return a/b;}
int ceil(int a,int b){if(a%b==0){return a/b;}return (a/b)+1;}
int gcd(int a,int b){int c;while(b!=0){c=a%b;a=b;b=c;}return a;}
int lcm(int a,int b){int c=gcd(a,b);a/=c;return a*b;}
int nCr(int a,int b){int i,r=1;for(i=1;i<=b;i++){r*=(a+1-i);r/=i;}return r;}
int nHr(int a,int b){return nCr(a+b-1,b);}
int fact(int a){int i,r=1;for(i=1;i<=a;i++){r*=i;}return r;}
int pow(int a,int b){int i,r=1;for(i=1;i<=b;i++){r*=a;}return r;}
long long llmax(long long a,long long b){if(a>b){return a;}return b;}
long long llmin(long long a,long long b){if(a<b){return a;}return b;}
long long llzt(long long a,long long b){return llmax(a,b)-llmin(a,b);}
long long llround(long long a,long long b){if((a%b)*2 >= b){return (a/b)+1;}return a/b;}
long long llceil(long long a,long long b){if(a%b==0){return a/b;}return (a/b)+1;}
long long llgcd(long long a,long long b){long long c;while(b!=0){c=a%b;a=b;b=c;}return a;}
long long lllcm(long long a,long long b){long long c=llgcd(a,b);a/=c;return a*b;}
long long llnCr(long long a,long long b){long long i,r=1;for(i=1;i<=b;i++){r*=(a+1-i);r/=i;}return r;}
long long llnHr(long long a,long long b){return llnCr(a+b-1,b);}
long long llfact(long long a){long long i,r=1;for(i=1;i<=a;i++){r*=i;}return r;}
long long llpow(long long a,long long b){long long i,r=1;for(i=1;i<=b;i++){r*=a;}return r;}
double dbmax(double a,double b){if(a>b){return a;}return b;}
double dbmin(double a,double b){if(a<b){return a;}return b;}
double dbzt(double a,double b){return dbmax(a,b)-dbmin(a,b);}
int sortfncsj(const void *a,const void *b){if(*(int *)a>*(int *)b){return 1;}if(*(int *)a==*(int *)b){return 0;}return -1;}
int sortfnckj(const void *a,const void *b){if(*(int *)a<*(int *)b){return 1;}if(*(int *)a==*(int *)b){return 0;}return -1;}
int llsortfncsj(const void *a,const void *b){if(*(long long *)a>*(long long *)b){return 1;}if(*(long long *)a==*(long long *)b){return 0;}return -1;}
int llsortfnckj(const void *a,const void *b){if(*(long long *)a<*(long long *)b){return 1;}if(*(long long *)a==*(long long *)b){return 0;}return -1;}
int dbsortfncsj(const void *a,const void *b){if(*(double *)a>*(double *)b){return 1;}if(*(double *)a==*(double *)b){return 0;}return -1;}
int dbsortfnckj(const void *a,const void *b){if(*(double *)a<*(double *)b){return 1;}if(*(double *)a==*(double *)b){return 0;}return -1;}
int strsortfncsj(const void *a,const void *b){return strcmp((char *)a,(char *)b);}
int strsortfnckj(const void *a,const void *b){return strcmp((char *)b,(char *)a);}

void shuffledget(int x[],int n){
    srand(time(0));
    int i,b[524288],p,c;
    for(i=0;i<n;i++){
        b[i]=i;
    }
    for(i=n;i>=1;i--){
        p=rand()%i;
        c=b[i-1];b[i-1]=b[p];b[p]=c;
    }
    for(i=0;i<n;i++){
        scanf("%d",&x[b[i]]);
    }
}

int main(void){
    int i,j,n,m,k,a[524288],b,c,w,r=0,l,t;
    double d;
    char s[524288];
    scanf("%d%d",&n,&k);
    //l=strlen(s);
    //for(i=0;i<n;i++){scanf("%d",&a[i]);}
    //shuffledget(a,n);
    //qsort(a,n,sizeof(int),sortfncsj);
    w=k-n-1;
    r=(w*(1+w))/2
    printf("%d\n",r-n);
    return 0;
}

Compiler output:
main.c:15:5: warning: conflicting types for built-in function 'round'; expected 'double(double)' [-Wbuiltin-declaration-mismatch]
   15 | int round(int a,int b){if((a%b)*2 >= b){return (a/b)+1;}return a/b;}
      |     ^~~~~
main.c:7:1: note: 'round' is declared in header '<math.h>'
    6 | #include<time.h>
  +++ |+#include <math.h>
    7 | #define inf 1072114514
main.c:16:5: warning: conflicting types for built-in function 'ceil'; expected 'double(double)' [-Wbuiltin-declaration-mismatch]
   16 | int ceil(int a,int b){if(a%b==0){return a/b;}return (a/b)+1;}
      |     ^~~~
main.c:16:5: note: 'ceil' is declared in header '<math.h>'
main.c:22:5: warning: conflicting types for built-in function 'pow'; expected 'double(double,  double)' [-Wbuiltin-declaration-mismatch]
   22 | int pow(int a,int b){int i,r=1;for(i=1;i<=b;i++){r*=a;}return r;}
      |     ^~~
main.c:22:5: note: 'pow' is declared in header '<math.h>'
main.c:26:11: warning: conflicting types for built-in function 'llround'; expected 'long long int(double)' [-Wbuiltin-declaration-mismatch]
   26 | long long llround(long long a,long long b){if((a%b)*2 >= b){return (a/b)+1;}return a/b;}
      |           ^~~~~~~
main.c:26:11: note: 'llround' is declared in header '<math.h>'
main.c: In function 'main':
main.c:71:18: error: expected ';' before 'printf'
   71 |     r=(w*(1+w))/2
      |                  ^
      |                  ;
   72 |     printf("%d\n",r-n);
      |     ~~~~~~