C :

#include<stdio.h>
int getMinGap(int c, int a1, int a2) {
	int t = a2 - a1;
	if (t < (c - t))
		return t;
	else
		return c - t;
}

int main() {
	int c, n;
	scanf("%d%d", &c, &n);

	int *const p = (int*)(malloc(n * (sizeof(int))));
	memset(p, 0, n);
	for (int i = 0; i < n; i++)
		scanf("%d", &p[i]);

	int maxgap;
	maxgap = getMinGap(c, p[0], p[n - 1]);
	for (int i = 0; i < n - 1; i++) {
		int gap = getMinGap(c, p[i], p[i + 1]);
		if (gap > maxgap)
			maxgap = gap;
	}

	printf("%d", c - maxgap);
	return 0;
}

C++ :

#include <bits/stdc++.h>
#define int long long
#define pb push_back
#define fer(i,a,b) for(int i=a;i<=b;++i)
#define der(i,a,b) for(int i=a;i>=b;--i)
#define all(x) (x).begin(),(x).end()
#define pll pair<int,int>
#define et  cout<<'\n'
#define xx first
#define yy second
using namespace std;
template <typename _Tp>void input(_Tp &x){
    char ch(getchar());bool f(false);while(!isdigit(ch))f|=ch==45,ch=getchar();
    x=ch&15,ch=getchar();while(isdigit(ch))x=x*10+(ch&15),ch=getchar();
    if(f)x=-x;
}
template <typename _Tp,typename... Args>void input(_Tp &t,Args &...args){input(t);input(args...);}
vector<pll> v;
const int N=1e6+10;
int a[N];
signed main()
{
    int sum,n;
	input(sum,n);
	int ans=0;
	fer(i,1,n){
	 	cin>>a[i];
        if(i>1) ans=max(ans,a[i]-a[i-1]);
	}
	ans=max(ans,sum+a[1]-a[n]);
	cout<<sum-ans; 
	return 0; 
}

Python :

# coding=utf-8
k,n=map(int,input().split())
a=[*map(int,input().split())]
a+=[k+a[0]]
print(k-max([a[i+1]-a[i] for i in range(n)]))