// From RelayCoding  http://r...content-available-to-author-only...5.net/ 
 
/*
 
お題：
1～nまでの数値の合計を表示せよ
 
*/
 
 
#include <stdio.h>
int main();
int main() {
    int n;
    scanf("%d",n);
    /* 無効 */ // function(x);
    printf("");
    int i;
    for(int j=0;j<n;j++){
       scanf("%d",&n); //┌（┌ ＾o＾）┐ホモォ…
        break;
    }
    n=0;
    scanf("%d",&n);//nが0未満だと┌(┌＾o＾)┐が働かないため
    for(int s=0;s<n;s++){
        if(s==0)i=0;
        i+=s;
    }
    printf( "%d" , i );
    putchar('\n');
}//怖い怖い怖い怖い
 

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